sustainability

Review Functionalized Carbon Nanotubes (CNTs) for Water and Wastewater Treatment: Preparation to Application

Mian Muhammad-Ahson Aslam 1, Hsion-Wen Kuo 1,*, Walter Den 2,*, Muhammad Usman 3 , Muhammad Sultan 4,* and Hadeed Ashraf 4

1 Department of Environmental Science and Engineering, Tunghai University, No. 1727, Section 4, Taiwan Boulevard, Xitun District, Taichung City 407, Taiwan; [email protected] 2 Department of Science and Mathematics, Texas A&M University—San Antonio, One University Way, San Antonio, TX 78224, USA 3 Institute for Water Resources and Water Supply, Hamburg University of Technology, Am Schwarzenberg—Campus 3, 20173 Hamburg, Germany; [email protected] 4 Department of Agricultural Engineering, Bahauddin Zakariya University, Multan 60800, Pakistan; [email protected] * Correspondence: [email protected] (H.-W.K.); [email protected] (W.D.); [email protected] (M.S.); Tel.: +886-(4)2359-0121 (ext. 3363) (H.-W.K.); +1-(210)784-2815 (W.D.); +92-333-610-8888 (M.S.)




 Abstract: As the world human population and industrialization keep growing, the water availability issue has forced scientists, engineers, and legislators of water supply industries to better manage Citation: Aslam, M.M.-A.; water resources. Pollutant removals from wastewaters are crucial to ensure qualities of available Kuo, H.-W.; Den, W.; Usman, M.; water resources (including natural water bodies or reclaimed waters). Diverse techniques have been Sultan, M.; Ashraf, H. Functionalized developed to deal with water quality concerns. Carbon based nanomaterials, especially carbon Carbon Nanotubes (CNTs) for Water nanotubes (CNTs) with their high specific surface area and associated adsorption sites, have drawn and Wastewater Treatment: Preparation to Application. a special focus in environmental applications, especially water and wastewater treatment. This Sustainability 2021, 13, 5717. critical review summarizes recent developments and adsorption behaviors of CNTs used to remove https://doi.org/10.3390/ organics or heavy metal ions from contaminated waters via adsorption and inactivation of biological su13105717 species associated with CNTs. Foci include CNTs synthesis, purification, and surface modifications or functionalization, followed by their characterization methods and the effect of water chemistry Academic Editors: on adsorption capacities and removal mechanisms. Functionalized CNTs have been proven to be Muhammad Sultan, Yuguang Zhou, promising nanomaterials for the decontamination of waters due to their high adsorption capacity. Redmond R. Shamshiri and Aitazaz However, most of the functional CNT applications are limited to lab-scale experiments only. Feasibil- A. Farooque ity of their large-scale/industrial applications with cost-effective ways of synthesis and assessments of their toxicity with better simulating adsorption mechanisms still need to be studied. Received: 24 April 2021 Accepted: 10 May 2021 Keywords: carbon nanotubes; surface modification; heavy metals; adsorption; water and wastewa- Published: 19 May 2021 ter treatment

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. Rapid urbanization and industrialization has significantly increased the clean water demands in the domestic, industrial, and agricultural sectors [1–3]. Meanwhile, large quantities of pollutants including organic, inorganic, and biological contaminants are being released into the water bodies from these sectors [4–6]. Eccentric waters such Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. as brackish, storm, and wastewater are being used depending upon the purposes [7,8]. This article is an open access article Increasingly, use of these waters has also increased the urgent concern about the burden distributed under the terms and of negative impacts on the surrounding environment; one of the tremendous challenges conditions of the Creative Commons confronting mankind is the exploration of green and sustainable methods to overcome Attribution (CC BY) license (https:// these shortcomings [9–13]. Keeping in mind the current situation of water and wastewater creativecommons.org/licenses/by/ treatment status, the technologies are not sustainable to meet healthy requirements for 4.0/). surrounding environment and community health [14,15].

Sustainability 2021, 13, 5717. https://doi.org/10.3390/su13105717 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 5717 2 of 54

Historically, numerous techniques and methods have been investigated for advanced treatment of water and wastewater [16]. The most common, adsorption, was proven to be the improved technique to remove a variety of pollutants including organic and inorganic contaminants present in water and wastewater [17,18]. Limited treatment effi- ciency was reported by using conventional adsorbents due their small surface area, limited number of active sites, deficiency in selectivity, and low adsorption kinetics [19]. These shortcomings of conventional adsorbents have been addressed in recent advancements of nano-adsorbents owing to their high surface area coupled with a higher number of active sites, tunable pore size, fast kinetics, and improved surface chemistry [10,20–24]. The nanomaterials can be used for treatment of water and desalination as well and also reveal properties including electron affinity, mechanical strength, and flexibility during functionalization [25–27]. Carbon nanomaterials (CNs) such as carbon nanotubes (CNTs) are supposed to be a promising material to break down the tradeoff concerning selectivity and adsorption, resulting in an increase of the economics of adsorption technology [25]. As a result, CNTs, as an adsorbent for treatment of water, have attained the focus of countless scholars over the previous few decades who are projected to carry on the exploration and developments in the field of CNs [28]. Numerous significant articles have been published on nanomaterials applied for the treatment of water and wastewater in previous few years [29–37]. Despite rapid developments, innovations, and applications of CNT-based nanomaterials, there is an increasing need for an across-the-board review of the synthesis of CNTs, functionalization of surface modifications, and finally their application to remove aqueous contaminants and to identify potential directions. This is the main motivation of the current review article. This review attempts to address a brief history of CNTs, synthesis, purification, and functionalization, followed by the application of these nanomaterials for eliminating or- ganics, inorganics, and microorganisms present in water and wastewater samples.

1.1. Historical Background The discovery of CNTs was reviewed in 2006 by Monthioux and Kuznetsov, showing that the science has seemed to remain controversial [38]. Most literature mentions that nan- otubes were discovered by Sumio Iijima [38]. However, Radushkevich and Lukyanovich explained the synthesis process of CNTs with 50 nm diameter [39]. Oberlin et al. explained the vapor phase growth technique for the synthesis of carbon fibers; the synthesized tubes consisted of turbostratic stacks of carbon layers (i.e., describing a crystal structure in which basal planes have slipped out of alignment) [40]. In addition, Abrahamson et al. [41] described the arc discharge method for carbon fiber synthesis using carbon anodes. Later on, scientists described the thermal catalytic disproportionation of CO for the synthesis of CNs. Transmission electron microscopy (TEM) and X-ray diffraction pattern (XRD) were used to characterize the synthesized CNs, and as a result they believed that CNTs can be formed by a graphene layer turning into a tubular shape. They also concluded that two types of promising arrangements, such as a helix-shaped spiral and circular arrangements in the form of a graphene hexagonal network, can result by turning the graphene layer into a tabular shape [42]. Later, a US patent was issued in 1987 on carbon nanofibers synthesis, the diameter ranging from 3.5 to 70 nm and five times greater in length than the diameter [43]. Back to the dates in 1950s, after the disclosure of CNTs by Iijima, projection of sur- prising properties of single-walled carbon nanotubes (SWCNTs) made by Dunlap and colleagues also attracted the attention of researchers around the world. At this time, after the discoveries and exploration of SWCNTs by Bethune and Iijima independently at IBM (Shiba, Minato) and Nippon Electric Co., Ltd. (Tokyo, Japan), respectively, the research on CNs and their specific methods of production was extended [44,45]. The above findings seem to be the extension of Fullerence’s discovery. Arc discharge technology had previously been applied for the production of laboratory-scale Buckminster Sustainability 2021, 13, 5717 3 of 54

fullerenes [46,47]. CNTs are being studied after the report published in 1991 by Iijima [45] was fundamental, because it put CNTs in the limelight [38].

1.2. Types of CNTs and Structure CNTs are composed of carbon atoms organized in a progression of fused benzene rings, which are pleated into a cylindrical shape. This new sort of man-made nano-material has a place with fullerene family and is treated as carbon’s third allotrope as well as sp2 and sp3 forms of graphite and diamond, respectively [42,48,49]. Generally, there are two types of CNTs [50] on the bases of number of layers shown in Figure1: 1. Single-walled carbon nanotubes (SWCNTs) 2. Multi-walled carbon nanotubes (MWCNTs).

Figure 1. Types of typical dimensions of CNTs, SWCNTs (left) and MWCNTs (right).

CNTs are made by a sheet of graphene when rolled into a cylindrical shape, which may have a capped or open end, usually in a hexagonal form close packed with a diameter at a small scale of 1 nm, and a few microns long. SWCNTs (Figure1) with a diameter as small as 0.4 to 2 nm are made by the single sheet of graphene rolled into a cylindrical shape, while MWCNTs (Figure1) with an outer and inner diameter ranging from 2–100 nm and 1–3 nm, respectively, and a several microns in length are made up of two or more sheets of graphene incasing a hollow core in the same way as in SWCNTs [49,51,52]. Based on the chemistry, there are two zones of CNTs: the sidewall and tip. A significant aspect in controlling these distinctive properties emanates after the change in the tube- like structure due to entrapment of graphene layers into a cylindrical shape. Figure2 shows different structures of rolled SWCNTs based on graphene sheets. Depending on alignment of the cylinder axis relative to the hexagonal matrix, the CNTs structure can be stipulated by chiral carrier in three ways, armchair, chiral, and zigzag, illustrated by their chirality index (n,m). Geometric arrangement of carbon atoms present at the layer of nanotubes is responsible for the foundation of zigzag (m = 0) and armchair (n = m) CNTs, whereas the structure of the nanotube with the two enantiomorphs on the right side is chiral (n 6= m) [53,54]. Sustainability 2021, 13, 5717 4 of 54

Figure 2. Roll-up of graphene sheet into different types of CNTs [54,55].

Recent reviews are good to find detailed elucidations of the structure of CNTs [48,49,56–61]. Here, Table1 summarizes a comparison of the properties of SWCNTs and MWCNTs.

Table 1. Comparison between properties of SWCNTs and MWCNTs [49,62,63].

Properties SWNTs MWCNTs Layer type Single graphene layer Multiple graphene layer Catalyst requirement Essential during synthesis No need during synthesis Bulk or massive production Difficult Easy Purity level Low Large Defect’s level High Low Characterization Easy Difficult Manage Easily twisted Cannot be twisted easily Specific gravity About 0.8 g/cm3 Less than 1.8 g/cm3 Elastic modulus About 1.4 TPa Ranging from 0.3 to 1 TPa Strength Ranging from 50 to 500 GPa Ranging from 10 to 60 GPa Electrical conductivity Ranging from 102 to 106 S/cm Ranging from 103 to 105 S/cm Electron mobility About 105 cm2/(V s) Ranging from 104 to 105 cm2/(V s) Thermal conductivity About 6000 W/(m K) About 2000 W/(m K) Coefficient of thermal expansion Greater than 1.1 × 10−3 K−1 About −1.37 × 10−3 K−1 Thermal stability in air Ranging from 600 to 800 ◦C Ranging from 600 to 800 ◦C Resistivity Ranging from 50 to 500 µΩ cm Ranging from 50 to 500 µΩ cm Specific Surface Area Ranging from 400 to 900 m2/g Ranging from 200 to 400 m2/g

2. Synthesis of CNTs Typically, there are three extensive methods for the synthesis of CNTs as given bellow: (a) Arc discharge (b) Laser ablation (c) Chemical vapor deposition (CVD). CNTs are produced by using energy and carbon sources in all the synthesis methods. A carbon electrode or a gas and an electric current or heat is used as a carbon and energy source, while using arc discharge or CVD methods, respectively, for the synthesis of CNTs, Sustainability 2021, 13, 5717 5 of 54

whereas a laser beam is used as an energy source during the laser ablation method. Table2 presents a detailed summary of the efficiencies of the CNT synthesis methods. These methods are based on the formation of a single or a consortium of carbon atoms that sack to recombine into CNTs. CNT synthesis mechanisms have been debated in detail by Cassell et al. [64] and Sinnott et al. [65]. It is believed that by using the metallic catalyst in the CVD method for CNT synthesis, the cylindrical shaped graphene tube is formed by initial deposition of carbon atoms on the used catalyst surface [66]. It was also concluded that the size of particles of the used catalyst also play an important role in the CNTs diameter, as the catalyst particles in a smaller size produce SWCNTs with a diameter of a few nanometers, while the larger particles tend to produce MWCNTs [67]. The arc discharge method between graphite electrodes is the first method of producing CNTs [68]. Briefly, in this method, direct current of 50 to 100 A and about 20 volts of potential difference is established between a graphite electrode pair in the presence of one of the inert gases containing helium or argon with a pressure of 500 Torr [69,70]. The carbon electrode surface evaporates and forms a cylindrical-shaped tube structure because of the high temperature generated due to the discharge of electric current in low pressure, inert gas, and catalyst [44,70,71]. MWCNTs can be synthesized via an arc discharge method without a metallic catalyst; on the other hand, mixed-metal catalysts, for example iron, cobalt, and nickel, are necessary for SWCNTs fabrication [72]. In general, higher levels of structural precision are noted in CNTs produced via the arc discharge method [73]; however, different variables such as chamber temperature, concentration and type of catalyst, hydrogen presence, etc., may affect the structure and size of synthesized CNTs [74]. Recently, nickel-filled CNTs were synthesized via a local arc discharge method in liquid ethanol [75], nitrogen-doped CNTs via vaporization of boron nitride [76], low-cost SWCNTs via an arc discharge method in open air [77], and SWCNTs and MWCNTs via a hot plasma arc discharge method [78]. The use of laser ablation to synthesize CNTs was first reported by Guo and colleagues in 1995 [79,80]. Briefly, a graphite object is targeted by a laser beam with high energy in the presence of argon at 800–1200 ◦C temperature and 500 Torr pressure [69,70,81,82]. In this method, a laser pulse provides an energy source, and a graphite object serves as a source of carbon. Soot deposition of carbon can be avoided by uniform evaporation of the target resulting because of continuous applications of laser pulses. The larger size particles after the first laser beam are broken down into smaller ones by the successive beams. Later, the smaller size particles are produced into the CNT structure. Commonly, transition metals are used as catalysts in this method. Rope-shaped CNTs can be found by using a laser ablation method with the diameter ranging from 10 to 20 nm and about 100 mm long [83]. CVD is a well-liked method for bulk fabrication of CNTs around the globe. Typically, in this method, carbon monoxide or hydrocarbons gases are used as a source of carbon, while 500 to 1100 ◦C of temperature is used as the source of energy. The carbon atoms are deposited by the decomposition of the used carbon source and shaped into CNTs [84,85]. Briefly, the decomposition of gas (carbon source) occurs because of high temperature when transferred into reaction chamber together with the carrier gas and generates a substrate of carbon atoms on the surface of catalyst to form CNTs [86]. When compared to other methods of synthesis of CNTs, the CVD is the most common route for relatively large-scale production of CNTs as it is simple in operation, higher in yield, and economic and has a high rate of deposition and good control over the morphology of tubes during the synthesis process [87–89] Cassell et al. [64] studied that CNTs in bulk can be produced, especially SWCNTs via the CVD method, by using acetylene as a source of carbon deposition in the presence of iron and cobalt and zeolite or silica as a carrier support material. They also concluded that SWCNTs can be produced on a largescale when a mixture of H2 and CH4 is deposited on the catalyst (Co or Ni), and MgO as carrier support material is used. Sustainability 2021, 13, 5717 6 of 54

Table 2. A summary of CNTs synthesis strategies and their efficiencies.

Laser Ablation Parameters Arc Discharge Method Chemical Vapor Deposition Ref. (Vaporization) Source of energy Direct current Temperature (ignition) High intensity laser beam Method Hydrocarbon gases or carbon monoxide [90,91] Source of carbon Carbon or graphite electrodes Graphite object (CH4, CO, or acetylene) Temperature (◦C) 3000 to 4000 500 to 1100 About 3000 [84,85] Cost per unit synthesis Costly Economic Costly [83,92] CNTs selectivity Less High Less [93] Availability of carbon source Complex Easy Difficult [94] Purification level More Less More [95] Nature of synthesis process Batch Continuous Batch [64,96] Control on synthesis parameters Difficult Easy Difficult [97,98] Energy requirement High Low High [99] Design of reactor Hard Simple and easy Hard [100] Nanotube graphitization High Moderate High [101–103] Typical yield 30 to 90% 20 to 100% Up to 70% [84,91,95,104–106] SWCNTs 0.6 to 1.4 nm 0.6 to 4 nm 1 to 2 nm [87,91,107,108] Typical Diameter Inner: 1 to 3 nm MWCNTs 0.1 to several nanometers 10 to 20 nm [79,83,91,104,109] Outer: ~10 nm Sustainability 2021, 13, 5717 7 of 54

Table 2. Cont.

Laser Ablation Parameters Arc Discharge Method Chemical Vapor Deposition Ref. (Vaporization) 1. Synthesis of both SWCNTs and MWCNTs is easy 1. Bulk production is easy 1. Primarily for SWCNTs 2. MWCNTs can be produced 2. More extensive length CNTs than 2. Diameter of CNTs can be without any catalyst other methods controlled 3. Costly process but less 3. Simple and easy process 3. Lower numbers of defects Advantages [98,110–113] than laser ablation method 4. Quite pure 4. High degree of structural 4. Synthesis of CNTs is 5. Alignment of produced CNTs is good perfection possible in open air 6. Diameter and number of layers can be 5. Tubes’ length can vary 5. High degree of structural controlled from 5 to 20 mm perfection 1. Received with some structural defects 2. Short and randomly 1. Costly technique due to distributed in length and 1. Only used to produce MWCNTs expensive lasers beams Disadvantages [65,86,98,109] direction 2. Higher structural defect density 3. Power needs are high 3. Lot of structural 4. Low yield purification is needed 4. Contains carbon impurities

Figures [114] Sustainability 2021, 13, 5717 8 of 54

3. CNT Purifications Some of the impurities include an amorphous phase of carbon, particles of partic- ular metals, or any other carrier material associated with CNTs that will have an effect on their execution performance [115]. Some typical purification technologies and their characteristics are discussed in Table3. On average, the CNTs synthesized via the CVD method showed a purity level ranging from 5% to 10% [116]. Therefore, a broad purifi- cation of CNTs is necessary before being used for different applications. The detection and identification of different impurities associated with CNTs using different techniques have been discussed in Table4. It is believed that the CNT structure may be affected to some extent when removing impurities, so there is always need for a compromise with the final structure after purification process [116]. The common CNTs purification methods are discussed below: 1. Oxidation 2. Acid treatment 3. Surfactant based sonication. Oxidation is a decent manner to remove carbon [117–122] and metal [117,121,123–126] impurities associated with CNTs. One of the main shortcomings that occur using this process of purification is the oxidation of CNTs themselves along with the impurities, but fortunately, the loss of CNTs is smaller than the impurities [114]. The reason to oxidize these impurities is more defects or open structures associated with them. This is another reason that the attachment of these impurities is often observed with a used metallic catalyst, and this metal catalyst may also play a role in oxidation [117,118,123,124,127]. There are some factors, such as type of oxidant, time of oxidation, temperature, metal contents, and environment, which can affect the oxidation efficiency and final yield. Typically, the method of acid treatment is used to eliminate the metallic impurities associated with CNTs. First of all, by oxidizing or sonication of the CNTs, the surface of associated impurities (metals) is made apparent to acid until the solvation and finally CNTs collect in suspension. It has been observed in a number of studies that by using HNO3 for the purpose of acid treatment, it only affects the metallic impurities rather than the CNTs or other carbon containing impurities [117,118,122,125]. By using HCl for this purpose, the impacts on CNTs and carbon impurities are also observed to a small extent [117,123,126]. Acid treatment for purification of CNTs in diluted form (4M HCl) can show same results as by the HNO3, but the metal surface must be apparent to the applied acid to make the solvation [128]. Although purified CNTs are produced relatively by acid reflux, the nanotubes amal- gamate, and the impurities that they capture are very hard or sometime impossible to remove by filtration [129]. Therefore, a surfactant-based sonication process is implemented generally by dissolving sodium dodecyl benzene sulfate (SDBS) in ethyl or methyl alcohol solution for this purpose. Since after sonication CNTs took longer to settle down, ultrafiltra- tion is required and then annealed at a high temperature (about 1000 ◦C) in the presence of N2 for 4 h. Annealing of CNTs is performed to optimize their structure. Surfactant-based sonication has been presented to be an effective method for removing tangled impurities associated with amalgamated CNTs [116]. Sustainability 2021, 13, 5717 9 of 54

Table 3. Typical purification technologies and their characteristics [130,131].

Characteristics Technologies Methods Advantages Limitations Filtration 1. Non-destructive 2. Retains the inherency and intrinsic structure necessary to elucidate the properties 1. Not very effective Centrifugation of CNTs 2. CNT samples need to be extremely Physical Solubilization with functional groups 3. More suitable as an auxiliary step in combination with chemical purification dispersed method 4. Improve crystallinity 3. Purification of samples can be done in High temperature annealing 5. High selectivity to metal a limited quantity at a time Chromatography, electrophoresis 6. CNTs can be separated on the bases of difference in length and conductivity Air, Cl ,H O, HCl, H O, Ar, Gas phase 2 2 2 1. Low yield O ,C H F , SF 1. Opens the lid of the CNTs without affecting sidewalls or associated functional groups 2 2 2 4 6 2. Produces more defects on sidewalls, 2. Eliminates polyhedral and amorphous carbon and metallic impurities at the cost of HNO ,H O , HCl, Mixture of breaks into different shorter length, and Chemical 3 2 2 substantial amounts of CNTs or damage to the CNT structure Liquid phase acid or KMnO , Microwave in also the alignment and structure are method 4 3. Leads to functional groups inorganic acid affected greatly, thus limiting the final 4. Does not disrupt or affect the alignments of CNTs applications of CNTs Electrochemical Alkali or acid solution Oxidation, sonication, centrifugation, filtration, wet grinding, and HIDE 1. High-purity with respect to metal Multi step Filtration/magnetic filtration, oxidation, annealing 2. Metal free, improving crystallinity 1. Low yield method 3. Effectively removes carbonaceous and metallic impurities Filtration, sonication in HNO3, HF, H2O2, or SDS 4. Better purification yield due to the early removal of metallic impurities that can oxidize CNTs Annealing at high temperature, extraction Sustainability 2021, 13, 5717 10 of 54

Table 4. Impurities associated with CNTs and their detection techniques.

Technique Residual Material Assessment Techniques Advantages Limitations After oxidation of material, the residual Thermo-gravimetric analysis Carbonaceous impurities metallic impurities are calculated by Completely oxidize/destroy the Accurate measurement of impurities (TGA) Metal impurities weighing ash and the carbonaceous CNTs impurities by area ratio of DTG Carbonaceous impurities Conductivity features and quality of The pure CNTs are associated with Difficult or even unacceptable for Raman spectrometry Structure defects CNTs can be measure along with G-band by RBM as well as no D-band MWCNTs and metallic contents Conductivity characteristics their diameter Directly observes and qualitatively Electron microscopy (SEM, Defects in CNTs evaluates the adhesion defects on the Absolute scrutiny can be Can analyze the sample in a very TEM) Amorphous carbon CNT wall, the amount of amorphous undertaken small amount carbon, fullerene Absorption spectroscopy or reflectance Carbonaceous impurities Conductivity features and contents A standard sample is needed with UV–vis-NIR spectroscopy in the ultraviolet-visible conductivity characteristics of CNTs can be analyzed exactly 100% purity spectral region Support material/functional groups Quantitatively characterizes the type X-ray photoelectron Analysis of functional groups on (fine alumina, magnesium oxide, and contents of functional groups or Unacceptable for purity spectroscopy (XPS) CNTs can be undertaken exactly silica, zeolite, etc.) support materials Energy-dispersive X-ray Analytical technique used for chemical Contents and traces of different Evaluation of the contents of CNTs Metal impurities spectroscopy (EDS) and/or elemental analysis of a sample elements can be analyzed is invalid Sustainability 2021, 13, 5717 11 of 54

4. Functionalization of CNTs The non-polar nature of graphene layers makes the CNTs naturally hydrophobic. The hydrophobic property of CNTs is indispensable for the adsorption of aromatic con- taminants like benzene and anthracene. A very strong complexion is formed due to π electrons present on the graphene layer making CNTs, between aromatic contaminants and the tube surface [132]. The surface affinity of CNTs can be modulated to a variety of contaminants in water and wastewater after the purification and surface functionalization. Higher adsorption of benzene was obtained by using CNTs as compared to activated carbon (AC) because of strong interaction between benzene rings and the surface of CNTs due its hydrophobicity [133]. Figure3 shows the different routes and schemes of CNT functionalization to increase their affinity for water and wastewater contaminates, which can be subsequently captured on the surface of CNTs used. Moreover, the functionalization of CNTs can be divided into three categories, shown in Figure4[134]: 1. With π conjugated network of CNTs through covalent bonds; 2. Attachment of different chemical groups via non-covalent bonds by using hydropho- bicity of CNTs such as hydrogen bonds, π–π interactions, or ionic bonds; 3. Inline filling (endohedral) of hollow tubes of CNTs. The two methods are more common for CNTs functionalization and variously used by the researchers.

Figure 3. Functionalization routes of CNTs and associated functional groups [132]. Sustainability 2021, 13, 5717 12 of 54

Figure 4. Methods of functionalization of carbon nanotubes.

CNTs are unique because of their distinctive properties such as adsorption capability, permeability, morphology, and physicochemical properties. There are several disadvan- tages that are also associated with raw CNTs, such as their low dispersion in solutions and low adsorption capacity for bulk fabrication of CNTs with organic and inorganic compos- ites. In fact, aggregation is the main problem for low contaminant adsorption efficiencies by the original CNT samples [33,53,135–137] and also obscures the process of membrane preparation [138]. The π–π interactions and van der Waals force between CNTs are respon- sible for the less dispersion, which results in tight fit bundles and aggregation of CNTs [139]. As the number of graphite layers of CNTs decreases from MWCNTs to SWCNTs, the ten- dency to bunch increases [140]. Therefore, to overcome these drawbacks, the chemical reactivity and contaminant adsorption capacity of CNTs must be improved by increasing their dispersion rate, and this is done by functionalizing the nanotube [53,135–137,139,141]. Furthermore, solubility of CNTs can be increased by their functionalization, which causes them to repel each other [50]. Table5 provides a compression between adoption capacities and the corresponding surface area of pristine and oxidized CNTs treated with different acids [142].

Table 5. Adsorption capacity of CNTs and corresponding surface area [142].

CNTs Adsorption Capacity (mg/g) Surface Area (m2/g) Pristine 1.1 82.2

H2O2 oxidized 2.6 130.0

HNO3 oxidized 5.1 84.3

KMnO4 oxidized 11.0 128. NaOCl oxidized 47.4 94.9

Different physical or chemical processes like oxidation, impregnation, or grafting (Figure3) are used for the functionalization of CNTs [ 143,144]. During the process of func- tionalization, the covalent or non-covalent bonds of particular functional groups result on the end or sidewall of CNTs. Functionalization is preferred over covalent bonding, because non-covalent bonding does not influence the structure and surface area of CNTs [135]. Advantages and the limitation of covalent and non-covalent functionalization of CNTs are discussed in Table6. Functionalization is generally done to increase the dispersibility as well as contaminant removal efficiency of CNTs, thereby improving the water or wastewa- ter treatment application capability of the CNTs or promoting membrane fabrication [145]. Higher dispersibility in polar solvent (water) was found by covalent functionalization of CNTs with the phenolic group by 1,3 dypolar cyclo addition; covalent functionalization of CNTs with phenol groups by 1, 3-dipolar cyclo-addition was found, which facilitated Sustainability 2021, 13, 5717 13 of 54

CNTs’ amalgamation into the polymer matrix [139]. Oyetade et al. [141] found that MWC- NTs efficiently absorbed Pb2+ and Zn2+ from aqueous samples after functionalizing with nitrogen. The increase in adsorption was due to higher surface area and more adsorption sites linked to CNTs.

Table 6. Benefits and limitations of covalent and non-covalent functionalization [134].

Methods Benefit(s) Limitation(s) Intrinsic characteristics are damaged Covalent Highly stable bonds are formed Structural defects CNTs functionalization Aggregation of CNTs Simple and easy procedure Non-covalent CNTs structure is maintained with minimum defects Stability of bonds is weak functionalization Electronic characteristics of CNTs are not affected

Oxidation of raw CNTs with HNO3,H2SO4, HCl, H2O2, KMnO4, and NaOCl, or sometimes a combination of some of these (Figure5), has often been exercised to introduce oxidized functional groups [146,147]. Generally, oxidation improves the dispersibility and enhances the ability to adsorb certain harmful contaminants in water and wastewater at the expense of fractional damage to the surface of CNTs, as described earlier. Furthermore, the surface of CNTs can also be modified with the addition of oxygen containing functional groups by performing oxygen–plasma action. In addition to oxidation and plasma action for surface modification, the CNTs can also be successfully modified with the addition of metal oxides like Al2O3 [148], MnO2 [149], and Fe3O4 [150], which deliver another way to coat the surface of CNTs, thereby increasing the contaminant removal efficiencies of CNTs [53,135–137]. The potential surface modification schemes of CNTs used for targeted contaminants are shown in Figure5.

Figure 5. Schematic representation of surface modification schemes of CNTs used for targeted contaminants (green arrows refer to decrease in final effluent concentration).

5. Characterization of CNTs Intrinsic properties of CNTs make them fascinating and desirable candidates for di- verse remediation. Characterization of CNTs plays an important role due to their distinctive properties. Numerous comments and debates have been published in past decades on Sustainability 2021, 13, 5717 14 of 54

different techniques and strategies used to evaluate CNTs [151–154]. Techniques used for the characterization of CNTs are divided into four groups: microscopy and diffraction, thermal, spectroscopic, and separation techniques [155]. Sometimes research includes one more group, the magnetic measurement technique. Table7 shows different characterization techniques used for the evaluation of CNTs. It must be noted more than one technique is prominent for the characterization of CNTs; techniques employed alone are not fully characterized, nor they are absolutely quantitative. Even though qualitative analysis of CNTs can be done by electron microscopy, scanning electron microscopy (SEM) evaluates the nanostructure of the tubes, and transmission electron microscopy (TEM) is used for further precise inspection, generally detecting the defects in CNTs [129,156].

Table 7. Different analytical techniques used for the characterization of carbon nanotubes [155].

Characterization Techniques Used for Studying Microscopy and diffraction techniques [157–159] SEM Morphological analysis (diameter and length), aggregation state Morphological analysis of internal structure (diameter, number of layers TEM/HR-TEM and distance between them) Morphological analysis of internal structure (diameter, number of layers AFM and distance between them) Morphological analysis of internal structure (diameter, number of layers Scanning tunneling microscopy and distance between them) Neutron diffraction Morphological analysis of bulk samples XRD Morphological analysis of bulk samples Spectroscopic techniques [139,160,161] Raman spectroscopy Purity and presence of by-products, diameter distribution, (n, m) chirality IR and FT-IR Purity, functionalization by attaching functional groups to the sidewall UV–vis and NIR Dispersion efficiency, diameter and length distribution, purity Fluorescence spectroscopy Size, dispersion efficiency, (n, m) chirality XPS and EDS Elemental composition, functionalization (covalent and non-covalent) Thermal techniques [162] Purity and presence of by-products, quality control of synthesis and TGA manufacture processes Separation techniques Size exclusion chromatography Purification, separation by size (length) Capillary electrophoresis Purification, separation by size (length, diameter, and cross-section) Field flow fractionation Fractionation by size (length) Ultracentrifugation Separation by chirality, electronic type, length, and enantiomeric identity Magnetic techniques [158,163,164] Vibrating sample magnetometry Magnetic properties Alternating gradient magnetometry Magnetic properties Superconducting quantum interference device Magnetic properties

The ultrastructure of different types of species including organic, inorganic, and biological species can be evaluated by using very popular techniques known as SEM and TEM. The scanning of the sample in SEM generates an image when the targeted electron ray interacts with sample of CNTs. Generally, the technique is used to analysis the morphology (length and diameter) of nanomaterials [123,155,165] to assess the quality of prepared CNTs. For example, Figure6a shows a SEM image of the as-prepared CNTs [ 148], and Figure6b Sustainability 2021, 13, 5717 15 of 54

shows alignment of CNTs synthesized by using a horizontal quartz tube housed in muffle furnace. Average length of the tubes was 70 µm measured by using SEM [166]. Sometimes, it is also used to validate the surface modifications in terms of functional reactions that occur on the surface of CNTs [167]. In the case where the required measurement exceeds 1 to 20 nm resolution while using SEM, then TEM is used [156,167]. Small dimensions in CNTs such as interlayer distance, diameter, and number of graphene sheets can be easily examined (Figure6c,d) by targeting the sample with a high energy electron beam of up to 300 keV [168,169]. It should be noted that the functional groups (organic and inorganic) that modify the surface of CNTs can also be evaluated by using TEM [161,170,171]. Moreover, structural integrity, surface functionalization, and defect son the surface of CNTs caused by the oxidation (acidic or basic) to introduce oxygen containing functional groups like hydroxyl, carbonyl, and carboxylic acid groups have also been studied by using both SEM and TEM techniques [155,172]. The image of the atomic structure and crystal structure information can be obtained by using high resolution transmission electron microscopy (HR-TEM) [173]. A high phase contrast image as small as the crystal unit can be obtained. This technology is widely used for advanced characterization of materials, allowing access to information on just-in-time defects, stacking faults, deposits, and grain boundaries. In addition to the morphology of the MWCNTs in the HR-TEM image, the direct measurements can be made on the MWCNT walls. For example, the number of walls constituting the nanotubes can be directly counted and recorded as control parameters in subsequent experiments in case the number of such walls needs to be changed. In addition, the interplanar distance between the walls can be accurately measured and compared with the crystal structure data table of the carbon structure and its respective diffraction pattern.

Figure 6. (a) SEM image of a bulk sample of multi-walled carbon nanotubes [155], (b) SEM image showing vertical aligned CNTs [174], (c) high-magnified TEM images of CNTs grown on unreduced catalyst [168] and, (d) TEM image of a bulk sample of multi-walled carbon nanotubes [155]. Sustainability 2021, 13, 5717 16 of 54

The chemical state and structure of CNTs can be obtained by using an X-ray photoelec- tron spectroscopy (XPS) technique [114]. However, the data obtained from this technique are used to examine the structural modification of CNTs before and after the chemical interactions with organics, inorganics, or gaseous adsorption. The investigation of CNTs by using XPS is done after the incorporation of nitrogen into the nanotube [175]. Due to the polar nature of the carbon–nitrogen bond, the peak shift before and after the modification is evidence of nitrogen incorporation [176,177]. Furthermore, the technology demonstrates that carbon nanofibers are more similar to carbon oxides than various graphites [178]. Fluo- rinated functionalization of SWCNTs was also studied by using XPS; the results concluded that three peaks of sp2, sp3, and carboxyl groups (284.3, 285, and 288.5 eV, respectively) were associated with C1s of un-doped nanotubes [114]. The observed carboxyl group (288.5 eV) was similar to in nanofibers [179]. Crystallinity of CNTs can be obtained by using an X-ray diffraction (XRD) tech- nique [90,176]. X-ray diffraction patterns of graphite and CNTs are very close to each other because of their inherent properties. The XRD pattern of CVD-synthesized MWCNTs is shown in Figure7, illustrating a peak similar to graphite (002), and the measured layer spac- ing can be obtained from Bragg’s law, while the other peaks (family of (hk 0) peaks)) can be obtained because the mono-graphene layer makes the honeycomb matrix [114]. Therefore, the curve obtained by XRD does not distinguish between the microstructure information of CNTs and graphite; nevertheless, it is helpful for purity analysis of the sample [102,103]. The XRD pattern revealed that straight CNTs with a good alignment on the surface of the substrate did not show the peak, i.e., 022 [114]. For CNTs aligned vertically to their substrate surface, the XRD pattern is not collected because of the scattering of the beams inside the sample. Therefore, the 002 peak is always lowered for better-aligned CNTs [180]. In addition, several other types of parameters such as bundle size, mean diameter, and diameter dispersion can also be studied by using the XRD technique. The peak 10 is greatly influenced by these parameters in terms of its location and thickness [181,182].

Figure 7. XRD pattern of MWCNTs synthesized by CVD [114].

Qualitative evaluation for nanotubes’ surface can be done by using Fourier transform infrared spectroscopy (FT-IR) [183]. The sample is characterized by passing the infrared radiations through it, and the part of radiations absorbed by the sample at specific energy is determined. Each functional group is identified by a particular range of frequency with associated absorption peaks. The infrared spectrum of the original CNTs (Figure8) shows a characteristic band of about 1600 cm−1 associated with aromatic rings (C=C bond) of Sustainability 2021, 13, 5717 17 of 54

rolled graphene layers. Sometimes, the peaks of 3800 to 3200 cm−1 (O-H stretch) and 1700 particles that absorb moisture into the atmosphere or due to certain purification processes [184]. In addition, a band of 2910 to 2940 cm−1 was also detected for CNTs, which is related to the vibration of C-H stretching methylene (CH2)[185]. The FTIR spectra can also suggest surface modification of CNTs. For example, in Figure8b, a new peak near −1 1450 cm appeared that was assigned to asymmetric CH2 bending. This peak is typically interpreted as evidence of defects in the structure of CNTs. In Figure8c, a new peak near 1735 cm−1 suggested a carbonyl stretch of the carboxylic acid group. In addition, a double peak at approximately 2900 cm−1 was attributed to the loss of aromaticity due to the oxygen functional groups.The thermal stability and proportion of volatile compounds of nanotubes can be analyzed by an analytical technique called thermogravimetric analysis (TGA). The analysis is made by heating the sample directly in the air or inert gases (He/Ar) while recording the change in its weight with respect to elevated temperature [176,186,187]. In some cases, the analysis is made in the presence of N2 or He with poor oxygen atmosphere (1% to 5% O2) to slow oxidation [188]. During the TGA analysis of the CNT sample in the air atmosphere, the weight loss of the sample (Figure9) is usually due to carbon oxidation to CO2, while the solid oxides after the oxidation of metallic catalyst are responsible for the superposition of the sample [165,189,190]. Generally, the percent yields of carbon deposits are determined by using TGA. Usually, during the oxidation of the sample the weight occurs in the temperatures ranging from 200 to 680 ◦C[190]. The contents of carbon can be calculated by obtaining the percentage of (m1 − m2)/m1, where m1 is the weight of the sample before oxidation and m2 is the weight of the sample after the oxidation [11].

Figure 8. FT-IR spectra of CNTs: (a) pristine CNTs (CNTs-WT), (b) CNTs treated with HCl (CNT-H),

and (c) CNTs treated with mixture of H3SO4 and HNO3 (CNTs-AC) [191]. Sustainability 2021, 13, 5717 18 of 54

Figure 9. TGA analysis of the CNT sample [190].

The structural features of CNTs can be defined by their crystalline structure, chiral carrier, and single- or multi-walled features [192]. Crystalline arrangement of nanotubes can be characterized by the ID/IG ratio determined from the RAMAN spectroscopy. ID/IG indicates the ratio between the organized and unstructured carbon in the CNTs and uses the intensity of defective and graphitic carbon (D and G bands) at the high wavenumbers in the RAMAN absorption spectrum (Figure 10)[193]. ID/IG is a good quality indicator for CNTs, and the low ID/IG ratio is characteristic of highly graphitized structures; the laboratory reported mass of MWCNT is 0.65, and industrial grade MWCNT is 2.04 [90,194].

Figure 10. Raman spectra of pristine (MWCNTs) and graphitized MWCNTs (g-MWCNTs) [193]. 6. Applications of CNTs 6.1. Removal of Heavy Metals Removal of heavy metals from water and wastewater by using surface modified CNTs has been studied extensively [143,195–197]. Contaminant adsorption mainly occurs at four possible types of sites that are available on the CNTs such as outer and inner grooves and interstitial channels mas shown in Figure 11, but the inner region of the nanotubes is less adsorbed [53,135–137,198]. Bahgat et al. [143] used functionalized MWCNTs for the adsorption of heavy metals and concluded that adsorption of the metals occurs because of a number of adsorption spots available on the tubes’ surfaces. In another study, higher adsorption of Zn2+ ions was observed by the plasma-treated CNTs due to the availability of more oxygen-containing functional groups. The mechanism of surface complexation was responsible for the adsorption of cationic ions, as the adsorption sites increased due to deprotonating functional groups [197]. Sustainability 2021, 13, 5717 19 of 54

Figure 11. Different adsorption sites on CNTs [198].

Surface modification of CNTs improved their affinity for heavy metal ions and metal- loid adsorption [199,200] by incorporating iron oxide and oxidation [16,201–203], coating with oxides of nonmagnetic metals [204], introducing a thiol functional group [82], and functionalizing with sulfur [205]. According to Addo Ntim and Mitra [203], the different oxides of iron such as magnetite (Fe3O4), maghemite (γ-Fe2O3), hematite (α-Fe2O3), and goethite (α-FeO) are very useful for the removal of trace heavy metals present in water and wastewater [170]. Generally, the process of surface modification of functionalization of CNTs is re- sponsible for the adsorption mechanisms of heavy metals [53,191,206]. Table8 shows an explanation of previously used functionalized CNTs and their adsorption capacities and removal mechanisms. Surface area, porous structure, functional groups, and interaction mechanisms between the absorbate and absorbents are the main attributes of CNTs for their heavy metal adsorption capability [53,136,137,143]. In addition, these properties enhance the heavy metal removal efficiencies onto the polymer film incorporated by functional CNTs [135]. Physical and chemisorption mechanisms of adsorption have been observed between the CNTs modified with metal oxides and heavy metal ions [203]. In addition, the contami- nant with particular hydration energy, ionic radius, and potential of hydrolysis also affects the adsorption capacities of CNTs. A study conducted by Hu et al. [207] confirms the above statement, when higher removal of Pb+2 was observed than Cu+2 by using MWCNTs modi- fied with iron oxide under the same experimental conditions. The adsorption performance of CNTs for heavy metals is also affected by the presence of other organics. For example, enhanced removal of Cd+2 ion was observed by using oxidized MWCNTs in the presence of 1–naphthol, while there was no effect recorded during the presence of Cd+2 ion on the removal of 1–naphthol in similar conditions [208]. These types of results mainly occur because of the distinct interaction mechanisms that are responsible for the adsorption of different types of pollutants [170]. Solution pH also plays an important role in the adsorption capacity and mechanisms of CNTs. For example, higher pH is favorable for endospheric interactions, while the lower pH facilitates extracellular interaction and/or ion exchange of targeted metal ions and surface functional groups of CNTs [208]. Moreover, as the pH increases (alkaline conditions), the charge density of functionalized CNTs moves towards more negativity, which is efficient for the adsorption of cationics, while, at lower pH (acidic conditions), because of protonating functional groups, the positive charge density increases, which repels the cationic metals, resulting in the lower efficiency of CNTs [53,136,137]. In a study, oxidized and ethylenediamine-doped MWCNTs were used for the adsorption of Sustainability 2021, 13, 5717 20 of 54

Cd+2 ions from aqueous samples. The results concluded that both types of absorbents removed Cd+2 ions strongly depending on pH ranging from 8 to 9 [209]. According to Rao and coworkers [146], the best adsorption capacity of nanotubes was observed in the pH ranging from 7 to 10. In addition to this particular range of pH, the ionization and competition between different species may also happen [210–213]. For example, an effective adsorption of Pb+2 ions on functionalized CNTs was reported during the co-existence of sodium dodecylbenzenesulfonate (DBS), while the adsorption of lead was significantly reduced in the presence of benzalkonium chloride [214]. The higher adsorption of lead ions might be due to anionic surfactant formation between Pb+2 and DBS, which are very complex compounds. In addition, the charge density (negative or positive) on the surface of CNTs makes the different interaction mechanisms of metal ions. For example, chemical interaction occurred between the N-dopped magnetic CNTs and Cr(III), while an electrostatic interaction was observed between the acid oxidized CNTs and Cr(III) [215]. Regardless of the costs, the CNTs are more efficient in terms of their adsorption and desorption phases than the AC, as reported in many studies [146,216]. Lu and cowork- ers [216] reported a slight decrease in adsorption and desorption of Ni+2 while using CNTs, and on the other hand, a sharp decrease was observed in the case of AC [216]. The adsorption of heavy metals by the functional CNTs depends on the interactions between available functional groups on the surface of CNTs and particular contaminants rather than the size of nanotubes. For example, higher adsorption of As(III) and As(V) was reported by Addo Ntim and Mitra [203] by using zirconia-modified CNTs of the same diameter (20 to 40 nm) than by using the CNTs coated with iron oxide. These results demonstrate higher arsenic adsorption by zirconia nanocomposites than the CNTs modified with iron oxide with similar range of diameters. Therefore, based on these results, further investigations on the adsorption capacities of functionalized CNTs are needed in terms of their surface area rather than the size CNTs. Sustainability 2021, 13, 5717 21 of 54

Table 8. Functional CNT based nanomaterials used for the removal of heavy metal ions from aqueous samples.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Tempkin, In column Liquid film Dubinin- operation, the MWCNTs 9.1 10–40 91% 6 40 µg/L 2.0 g/L diffusion, ion Radushkevic, removal As(III) [217] exchange Langmuir, was up to Freundlich 13.5 µg/L Ion Zero-valent Maximum As(III) exchange, iron-doped - - 200 mg/g 4 10 mg/L 0.2–4.0 g/L Langmuir removal efficiency [218] surface com- MWCNT was 98.5% plexation Floating catalyst 74 18.6 1.22 mg/g CNTs (FCNT) Electrostatic Potential Oxidized-FCNT 129 10.7 1.90 mg/g attraction, adsorbent for 6.5 0.1–10 mg/L 1 g/L Langmuir [219] Heat-treated surface com- removal to total As(III) oxidized CNTs 168 7 5.99 mg/g plexation arsenic (FCNT-HOX) Zero-valent iron Reusability of Surface com- immobilized on 78.78 111.1 mg/g 7 0.1–1 mg/L 0.05 g Langmuir adsorbent was up [219] plexation MWCNTs to 5 cycles The adsorption Chemisorption/ capacity of AS (III) MWCNT-ZrO 152 20–40 2 mg/g 6 100 µg/L 100 mg/10 mL physisorp- Langmuir [203] 2 is not associated tion with pH value Suggesting that modifying Electrostatic Iron-oxide-coated MWCNTs with interaction, multi-walled 153 20–40 1.723 mg/g 4 100 µg/L 10 mg/10 mL Langmuir other groups can [203] surface com- carbon nanotubes develop potential plexation adsorbents for water treatment Sustainability 2021, 13, 5717 22 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Adsorption was Iron-oxide-coated Surface com- very fast for low - - 49.65 mg/g 4 5–50 mg/L - Freundlich [220] SWCNTs plexation concentration of As(V) Tempkin, In column Liquid film Dubinin- operation, the MWCNTs 9.1 10–40 92% 6 40 µg/L 2.0 g/L diffusion, ion Radushkevic, removal As(III) [217] exchange Langmuir, was up to Freundlich 14.0 µg/L Ion Maximum As(V) Zero valent iron exchange, - - 250 mg/g 4 10 mg/L 0.2–4.0 g/L Langmuir removal efficiency [218] doped MWCNTs surface com- was 98.5% plexation Floating catalyst 74 18.6 0.88 mg/g CNTs (FCNT) As(V) Electrostatic Potential Oxidized-FCNT 129 10.7 2.51 mg/g attraction, adsorbent for 6.5 0.1–10 mg/L 1 g/L Langmuir [219] Heat-treated surface com- removal to total oxidized CNTs 168 7 6.37 mg/g plexation Arsenic (FCNT-HOX) 3-(2- Cr(IV) was aminoethylamino) selectively propy- adsorbed in the - - 8.01 mg/g 2.2 1.0 mg/L 40 mg - - [221] ltrimethoxysilane micro-column modified packed with MWCNTs adsorbent Successfully Zero-valent iron Surface com- applied to ground immobilized on 78.78 167 mg/g 7 0.1–1 mg/L 0.05 g Langmuir [219] plexation water with high MWCNTs pH Sustainability 2021, 13, 5717 23 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Iron(III)-oxide- Greater efficiency coated to remove As(V) ethylenediamine 198.5 5–10 23.5 mg/g 4 100 µg/L 50 mg/10 mL Ion exchange Langmuir [222] due to enormous functionalized adsorbing sites MWCNTs The adsorption Chemisor- MWCNT–zirconia capacity of As(V) 152 20–40 5.0 mg/g 6 100 µg/L 100 mg/10 mL ption/ ph- Langmuir [223] nanohybrid is not associated ysisorption with pH value Modifying Electrostatic MWCNTs with Iron-oxide-coated interaction, other groups can 153 20–40 0.189 mg/g 4 100 µg/L 10 mg/10 mL Langmuir [203] MWCNTs surface com- develop potential plexation adsorbents for water treatment Iron oxide/carbon Decrease in nan- efficiency was 12% otubes/chitosan 64.4 66.25 mg/g 2–10 100 mg/L 0.3 mg/g Electrostatic Langmuir after reusing the [224] magnetic adsorbent for ten composite film cycles 10-fold greater Cr(III) Nitrogen-doped Chemical removals than magnetic carbon - - 83.7 mmol/g 8 200 mg/L 10 mg/500 mL Langmuir [225] adsorption activated carbon nanoparticles due to large SSA Increasing Pseudo- Acid modified Electrostatic removal of Cr with - 23 0.5 mg/g 7 1 mg/L 120 mg/500 mL second [215] MWCNTs interaction increasing the order dose of CNTs Sustainability 2021, 13, 5717 24 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Iron oxide/carbon Decrease in nan- efficiency was 6% otubes/chitosan 64.4 - 449.3 mg/g 10-Feb 100 mg/L 0.3 mg/g Electrostatic Langmuir after reusing the [224] magnetic adsorbent for ten composite film cycles Recycled adsorbent was Nitrogen-doped 40–1000 Surface com- successfully used 116.4 - 970.9 mg/g 1 0.5–3.5 g/L Langmuir [163] magnetic CNTs mg/L plexation for excellent electrochemical reduction of CO2 Cr(IV) Chitosan- Adsorbent can be modified - 30–50 164.0 mg/g 2 50 mg/L 50 mg Electrostatic Langmuir recycled up to 4 [226] MWCNTs times Absorbent highly showed durability, Magnetic iron - ~50 42.0 mg/g 2 5 mg/L 0.4–1.0 g/L Electrostatic Langmuir selectivity, easy [227] oxide MWCNTs regeneration ability Removal of Cr(IV) was enhanced Chitin magnetite 69.1 - 100% 2 50 mg/L - Physical - after mixing [228] MWCNTs MWCNTs with chitin Sustainability 2021, 13, 5717 25 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism The adsorption capacity of adsorbent increases with initial Magnetic 200 20–40 16.23 mg/g 3 25 mg/L - - Langmuir concentration of [229] MWCNTs Cr(VI) and contact time, but decreases with the increase of adsorbent dosage Cr(IV) was 3-(2- selectively aminoethylamino) adsorbed in the propyltrimethoxysilane- - - 9.79 mg/g 2.2 1.0 mg/L 40 mg - - [221] micro-column modified packed with MWCNTs adsorbent The f-CNT can be Activated-carbon- used largely for coated - 10–20 9.0 mg/g 2 0.2–0.5 mg/L 2 mg/50 mL - Langmuir [215] the removal of Cr CNTs ions Suggesting that Ceria-supported CeO2/ACNTs has CNTs - 20–80 31.55 mg/g 7 35.3 mg/L 100 mg/100 mL Ion exchange Langmuir high potential for [230] nanoparticles heavy metal removals Sustainability 2021, 13, 5717 26 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism The adsorbent Thiol- Lewis removed heavy functionalizedM- 97.367 - 65.4 mg/g 6.5 50 mg/L 100 mg/100 mL acid–base Langmuir metal ions [231] WCNTs/Fe3O4 interactions effectively at various pH values High removal efficiency due to Magnetic intrinsic 295.4 - N/A 6 100 mg/L 1000 mg - Experimental [232] MWCNTs properties, large SSA, and porous structure Easily regenerate Electrostatic, the adsorbent by hydrophobic, MWCNTs/Fe O 108.37 10–20 22.04 mg/g 5.3 30 mg/L 500 mg/1000 mL Langmuir external magnetic [233] Pb(II) 3 4 and π–π field after several interactions cycles Easily regenerate MWCNTs/Fe O Electrostatic, 3 4 the adsorbent by modified with 3- hydrophobic, 90.68 10–20 75.02 mg/g 5.3 30 mg/L 500 mg/1000 mL Langmuir external magnetic [233] aminopropyltrie- and π–π field after several thoxysilane interactions cycles The f-CNT membrane MWCNTs potentially Electrostatic grafted/PAAM - - 98% - 10 mg/L 1000 mg/1000 mL - enhances the [234] interaction membrane water flux and removal of heavy metals Sustainability 2021, 13, 5717 27 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Considering the oxidize CNT Oxidized CNT Chemical - - 117.65 mg/g 7 1200 mg/L 50 mg/25 mL Langmuir sheets promising [235] sheets interaction nanomaterial for adsorption Showed high suitability for Ion MWCNTs grafted preconcentration exchange, with - - 37.0 mg/g 6 10 mg/L 640 mg/1000 mL Langmuir and [236] electrostatic 2-Vinylpyridine immobilization of interaction heavy metal ions from water Chemical, High removal electrostatic, Oxidized 0.021 efficiency toward 142.29 10–30 4.1 0.83 mmol/L 0.75 g/L hydrophobic, Langmuir [214] MWCNTs mmol/g heavy metal ions and π–π in wastewater interactions The composite can be used largely to remove lead from industrial Alumina-coated - - 99% Different - 10 mg/25 mL N/A - wastewater. [236] MWCNTs Adsorption efficiency increased with the pH (3 to 7) Nitrogen-doped High removal Chemical magnetic carbon - 6.74 mmol/g 8 200 mg/L 10 mg/500 mL Experimental efficiency toward [225] adsorption nanoparticles Pb compared to Cr Sustainability 2021, 13, 5717 28 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Important Titanium Dioxide adsorption ability /MWCNT - - 137.0 mg/g 6 10 mg/L 20 mg/10 mL - Langmuir to remove large [204] composites amount of Pb(II) in short period The sorption of Pb largely depends Pb(II) Oxidize MWCNTs - 20–30 - - 10 mg/L 3000 mg/1000 mL - - [237] on foreign ions and ionic strength Electrostatic 300% greater Manganese interaction, adsorption oxide-coated 275 2.60 78.74 mg/g 5 30 mg/L 50 mg/100 mL Langmuir [238] surface com- capacity than raw CNTs plexation CNTs The regeneration of Pb increasing Acidified Physical with decreasing 237.3 29.0 85 mg/g 5 50 mg/L 25 mg/50 mL Langmuir [239] MWCNTs adsorption pH and can be used for several cycles Electrostatic Capable of interaction, Alumina- removing both physical Cd(II) decorated 109.8 10–20 27.21 mg/g 7 1 mg/L 50 mg/L Langmuir metallic and [240] adsorption, MWCNTs organic surface Contaminants precipitation Sustainability 2021, 13, 5717 29 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism The sorption capacity is strongly dependent on pH Oxidized due to surface 78.5 16.09 24.15 mg/g - 5 mg/L 1 mg/10 mL Chemisorption Langmuir [209] MWCNTs charge and showed best performance in the pH ranging from 6 to 10 The sorption capacity is strongly dependent on pH Ethylenediamine- due to surface functionalized 101.2 21.25 25.7 mg/g - 5 mg/L 1 mg/10 mL Chemisorption Langmuir [209] charge and MWCNTs showed best performance in the pH ranging from 6 to 10 Excellent removal Oxidized CNT Chemical - - 92.59 mg/g 7 1200 mg/L 50 mg/25 mL Langmuir of heavy metal [235] sheets interaction ions Potential material Acid-modified Electrostatic 170 10–20 4.35 mg/g 7 - 50 mg Langmuir for water [241] CNTs interaction purification The removal efficiency MWCNTs Electrostatic increases with modified with - 60–100 - - - 2000 mg - [242] interaction increase of mass of Chitosan both MWCNTs and chitosan Sustainability 2021, 13, 5717 30 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Higher adsorption MnO -coated Electrostatic affinity to other 2 110.4 30–50 58.82 mg/g 5–7 10 mg/L 200 mg/20 mL Langmuir [149] CNTs interaction heavy metals rather than Hg Easily des- Thiol-derivatized Electrostatic orb/regenerate Hg - - 131.58 mg/g 5 40 mg/L 0.25 mg/mL Langmuir [243] SWCNTs interaction after treatment of water Highly efficient Amino and removal from real thiolated wastewater and - 5–10 84.66 mg/g 6 100 mg/L 60 mg Physisorption Langmuir [244] Hg(II) functionalized- further research is MWCNTs necessary to commercialize Successfully used Iodide- 100–500 for the adsorption incorporated 153 10–20 123.45 mg/g 6 2500 mg/1000 mL Ion exchange Langmuir [205] mg/L and desorption of MWCNT (CNT-I) Hg(II) Greater treatment ability for Sulphur- industrial containing - - 72.8 µg/g 12.15 0.1mg/L 100 mg/20 mL Chemisorption Freundlich wastewater [245] CNTs containing Hg and other anions and cations Better removal of Thiol- Lewis heavy metals in functionalized- 97.2 - 65.52 mg/g 6.5 50 mg/L 1000 mg/100 mL acid–base Langmuir [231] Hg(II) different pH MWCNTs/Fe O interactions 3 4 concentration Sustainability 2021, 13, 5717 31 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Small diameter of CNTs removing Oxidized Electrostatic - - 3.83 mg/g 7 10–100 µg/L 25 mg/50 mL Langmuir greater amount of [246] MWCNTs interaction Hg(II) from aqueous solution Excellent potential Functionalized Electrostatic 250 10–25 2.42 mg/g 10 1.1 mg/L 0.09 g Langmuir for the removal of [247] MWCNTs interaction heavy metal ions Economically feasible material with excellent Chemical Oxidized CNTs - - 74.63 mg/g 7 1200 mg/L 50 mg/25 mL Langmuir heavy metal ion [235] interaction removal efficiency without any CNTs leakage Zn(II) The removal efficiency Chitosan- Electrostatic increases with - 60–100 N/A 7 - 200 mg N/A [242] MWCNTs interaction increase of mass of both MWCNTs and Chitosan Higher specific surface area and Nitrogen-doped Chemical nitrogen make the magnetic carbon - - 9.31 mmol/g 8 12.82 mg/L 10 mg/500 mL Langmuir [225] adsorption nanomaterial an nanoparticles excellent adsorbent Further research is Oxidized Electrostatic necessary to - 14 0.27 mmol/g 6.5–6.8 15 mg/L 5 mg/5 mL Langmuir [199] Zn(II) MWCNTS interaction understand the full mechanism Sustainability 2021, 13, 5717 32 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Zinc ion could be Sodium- easily regenerated, hypochlorite- Electrostatic - <10 34.36 mg/g - 60 mg/L 50 mg/100 mL Langmuir and the adsorbent [211] treated interaction can be used for MWCNTs many cycles Enabling CNTs for Electrostatic wastewater Sulfonated interaction, treatment and 28.7 - 43.16 mg/g 5 20 mg/L 25 mg/50 mL D–R model [248] MWCNTs surface com- composite plexation formation or physical blending Electrostatic Easily regenerate Magnetic interaction, the Cu after - 10–20 38.91 mg/g 30 mg/L 200 mg/1000 mL - [232] MWCNTs physical removal from interaction polluted water Considering the oxidize CNT Oxidized CNT Chemical sheets promising - - 64.93 mg/g 7 200 mg/L 50 mg/25 mL Langmuir [235] Cu(II) sheets interaction nanomaterial for heavy metal adsorption No loss in the Chitosan/ adsorption poly(vinyl) - 5–20 11.1 mg/g 5.5 30 mg/L 0.5–2 wt% Ion exchange Langmuir capacity after four [249] functionalized regeneration MWCNTs cycles The removal efficiency MWCNTs Electrostatic increases with modified with - 60–100 >95% - - 2000 mg - [242] interaction increase of mass of Chitosan both MWCNTs and chitosan Sustainability 2021, 13, 5717 33 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Effective preconcentration Chitosan-grafted - - 24.0 mg/g - 10mg/L 1000 mg/1000 mL N/A - and solidification [250] MWCNTs of heavy metals in aqueous samples Poly(acrylic Promising ability 1.66 × 10−4 1.69 × 10−4 Surface com- acid)-grafted -- 6.8 1.0 g/L Langmuir to use in water [251] mol/g mol/L plexation MWCNTs purification Highlights the Ion interaction MWCNT / iron exchange, between heavy S - - 0.18 mmol/g 6.4 4.2 mg/L 0.5 g/L Langmuir [252] oxide surface com- metals and organic Co(II) plexation substances in wastewater Considering the oxidized CNT Oxidized CNT Chemical sheets promising - - 85.74 mg/g 7 1200 mg/L 50 mg/25 mL Langmuir [235] sheets interaction material for the removal of heavy metal ions Better removal HNO -treated 3 102 10–20 17.86 mg/g 6.5 20 mg/L 0.8 g/L Ion exchange Langmuir efficiency toward [253] MWCNTs heavy metal ions The removal efficiency MWCNTs Electrostatic increases with an Ni(II) modified with - 60–100 90% - - 2000 mg - [242] interaction increase of mass of Chitosan both MWCNTs and Chitosan The removal Nitrogen-doped Chemical efficiency was not magnetic carbon - - 8.06 mmol/g 8 12.82 mg/L 10 mg/500 mL Langmuir [225] adsorption very good for Ni nanoparticles compared to Cr Sustainability 2021, 13, 5717 34 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Effective Electrostatic Poly(acrylic acid) preconcentration 6.615 × 10−6 interaction, (PAA)-oxidized 197 - 5.4 5 mg/L 0.8 g/L Langmuir and solidification [60] mol/g π–π MWCNTs of Ni(II) in liquid interaction samples High removal affinity to heavy NaClO-modified Electrostatic - 380 47.86 mg/g 7 10–80 mg/L 50 mg/100 mL Langmuir metals and can be [216] SWCNTs interaction used for water treatment Promising candidate for the solidification and MWCNTs/ Iron preconcentration - - 9.18 mg/g - 6 mg/L 0.75 g/L Ion exchange Langmuir [201] oxide of heavy metal ions as well as for radionuclides from water Greater adsorption Oxidized Electrostatic - 5.5–14 49.26 mg/g - 10–200 mg/L 20 mg/50 mL - ability than raw [254] MWCNTs interaction MWCNTs in water Excellent material Oxidized Electrostatic 197 10–30 >80% 8 6–20 mg/L 50 mg/200 mL - for the adsorption [255] MWCNTs interaction of metal ions. Ion exchange, Excellent sorption surface com- of Ni+2 ions with Ni(II) MWCNTs 40–600 40–60 6.09 mg/g 7 25 mg/L 5 g/L - [256] plexation, smaller chemical equilibrium time interaction Sustainability 2021, 13, 5717 35 of 54

Table 8. Cont.

Surface Diameter Experimental Conditions Removal Adsorbate Adsorbent 2 Qe/RE Model Comments Ref. Area (m /g) (nm) pH IC AL Mechanism Adsorption Diglycolamide- efficiency functionalized increased with the U(II) 300–600 - 133.74 mg/g 7 - 1–10 mg - Langmuir [257] MWCNT increasing dose of (DGA-MWCNTs) adsorbent and temperature Adsorption efficiency Oxidized- Diffuse layer increased with Sr(II) - - 36% 2-11 - 3 g/L - [258] MWCNTs model increasing pH but decreased with the ionic strength Higher adsorption Oxidized- Diffuse layer Eu (III) - - 96% 2-11 - 3 g/L - efficiency for [258] MWCNTs model Eu(III) than Sr(II)

Qe = Max adsorption capacity (mg/g); RE = Removal efficiency; IC = Initial concentration; AL = Adsorbent loading; - = Data not available Sustainability 2021, 13, 5717 36 of 54

6.2. Removal of Organics Organics due to human actions [259], animals, or plant deterioration [260] are present in water and wastewater, usually in the form of dissolved and/or suspended particulate matters [135]. Zare et al. [261] reported that the adsorption efficiencies of CNTs for or- ganic dyes can be enhanced after the functionalization. Methylene blue and orange were effectively removed from the water matrix by using oxidized MWCNTs [262,263] as com- pared to other types of modified adsorbents. Duman et al. [264] successfully used a novel nanocomposite, MWCNTs/carrageenan/Fe3O4, for effective removal of crystal violet. New nanocomposites can also be used for CNT modification to increase the adsorption capacity of cationic dyes. Sadegh and coworkers [265] reported that the adsorption of amide black can be significantly enhanced by using MWCNT-COOH-cysteamine. The results showed a maximum adsorption capacity of 131 mg g−1 for MWCNT-COOH-cysteamine, while it was 90 mg g−1 for MWCNT-COOH [265]. The adsorption capacities of CNT composites can be reduced effectively because of competition that occurred between several types of organic contaminants present in water and wastewater at the same time [266], similarly to as described for heavy metal adsorption. However, as reported by Ali et al. [135], the initial concentration did not have any significant effect on the removal capacities of CNTs for the removal of inorganic contaminants. As a result, customization and modifications of the surface of CNTs make them significant for selective adsorption of organic contaminants. Moreover, Wang et al. showed about 95% efficiency of the CNT composite for pharmaceutical and personal care product (PPCP) removal by increasing the specific surface area and aromatic ring of CNTs [267,268]. Jahangiri-Rad et al. [269] showed an adsorption capacity of 496 mg g−1 by using oxidized SWCNTs with large specific surface for the removal of blue 29 dye. Adsorption capacities and mechanisms responsible for the interaction between or- ganic contaminants and functionalized CNTs at different experimental conditions are described in Table9. The increase in oxygen-containing functional groups on the surface of nanotubes results in a decrease in natural organic maters (NOM), because the π–π interaction decreases due to more electrostatic repulsion [270]. The same type of results were shown in the case of higher pH [270]. Therefore, π–π interactions are responsible between the large specific surface area of functionalized CNTs and NOM for the adsorption area of CNT [271,272]. Yang et al. [208] also reported similar results, as organics may be absorbed by π–π interactions occurring between the surface of CNTs and aromatic rings of 1–naphthol. CNTs have also been effectively used for the removal of pesticides [170]. Oxidized and as prepared MWCNTs showed excellent removal efficiency for diuron at pH ≥ 7.0 [273]. Oxidation of CNTs leads to the increase of their surface area, and pore size results in the higher adsorption of diuron [170]. Hamdi et al. [274] reported a reduction in uptake of chlordane and p,p0-dichlorodiphenyldichloroethylene from 78% to 23% in the roots of a lettuce crop by the addition of CNTs functionalized with amino group. The pesticides (1- pyrenebutyric, 2,4-dichlorophenoxyacetic, and diquat dibromide) were absorbed more (up to 70.6%) on semiconductor CNTs than metallic CNTs [275]. The lower density of electrons on the surface of semiconductor CNTs were found to facilitate higher adsorption [275]. On the other hand, during the batch modes, the pesticide removal was found to be limited [276]. SWCNTs and MWCNTs were used for the removal of diquat dibromide in the fixed bed system. The results were not higher than those of the batch system, but the pesticide absorbed completely in the fixed bed system by increasing the time [276]. The CNTs are also being used for nano-filtration of contaminants from aqueous so- lutions [162,277,278]. For adsorbents, the selectivity of nano-filters can be controlled by attaching different functional groups to the surface of CNTs [279]. Beside the hydrophobic- ity of CNTs, molecular dynamics simulations showed a weak interaction between water and nanotubes [280]. Hummer [281] explained that friction-free water flow was caused by the nano-scale pore size, which makes interaction energy smaller and finally lowers the interactions with water [281]. Sustainability 2021, 13, 5717 37 of 54

Table 9. Functionalized CNTs used for organic pollutants removal.

Dye Surface area Removal percentage Optimum Adsorbent Q (mg/g) Remarks Ref. Pollutants (m2/g) (%) conditions pH 9, IC = 150 mg/L, Exothermic process favored at AL = 0.05 g, Contact lower temperature range Oxidized SWCNT Basic red 46 (BR 46) 400 49.45 - [282] time = 100 min, Orderly adsorption of dye due 298 K, to negative entropy Endothermic process of pH 1, IC = 30 mg/L, HNO3-oxidized Bromothymol blue adsorption significantly affected 96.8 55 97 AL = 0.02 g, [262] MWCNTs (BTB) by pH, initial concentration, T = 293.15 K, sorbent dosage, and contact time Strong electrostatic interactions Functionalized pH 7, AL = 30 mg, between dye particles and Congo red 148 1250 94 [283] CNT/Mg(Al)O contact time = 75 min functional groups associated with the surface of nanomaterial Magnetic pH 3.7, IC = 54 mg/L, Spontaneous endothermic MWCNTs-Fe3C Direct red 23 38.7 85.5 - AL = 0.04 g, adsorption process due to [284] nanocomposite T = 333 K, positive enthalpy pH 7, IC = 10 mg/L, SWCNT–COOH Malachite Green 400 22.33 - Adsorption significantly affected 300 K, by ionic strength, initial [285] pH 7, IC = 10 mg/L, concentration, sorbent mass, SWCNT-NH2 Malachite Green 400 29.36 - T = 300 K, contact time, and temperature More active functional groups pH 7, IC = 10 mg/L, SWCNT–COOH Methyl orange 400 25 - on SWCNT-NH2 adsorbed more T = 300 K, dye than SWCNT-COOH pH 7, IC = 10 mg/L, SWCNT-NH2 Methyl orange 400 27.15 - T = 300 K, Initially, rapid adsorption was observed, but it slowed down with the time AL = 20 mg/L, As the mixture temperature, Oxidized MWCNTs Methyl orange 165 10 - T = 313 K, stirring [286] agitation speed, and initial speed = 500 rpm concentration increased, the adsorption efficiency also increased Sustainability 2021, 13, 5717 38 of 54

Table 9. Cont.

Highly active hydroxyl and Functionalized- pH 6.5,IC = 5 mg/L, amine functional groups made CNTs loaded Methyl orange - 42.85 100 contact time = 30 min, [287] TiO -CNT composite an TiO T = 298 K 2 2 effective adsorbent pH 6,IC = 10 mg/L, As the temperature and initial Thiol-functionalized AL = 20 mg, concentration increased, the MWCNT Methylene blue 400 166.67 - [288] T = 298 K, Contact adsorption efficiency also (MWCNT-SH) time = 60 min, increased Phenol and its Surface area Removal percentage Optimum Adsorbent derivatives Q (mg/g) Remarks Ref. (m2/g) (%) conditions pollutants pH 6, IC = 40 mg/L, KOH-modified Bisphenol-A 0.20 mmol/g 494.48 - contact time = 5 min, As the pH increased, the MWCNTs [289] T = 298 K, adsorption capacity decreased pH 6, IC = 40 mg/L, because of deprotonating; both HNO -modified 3 Bisphenol-A 0.59 mmol/g 153.79 - contact time = 30 min, the negatively charged function MWCNTs T = 298 K, groups and adsorbates repel each other SOCl /NH OH- pH 6.5, IC = 10 mg/L, 2 4 Adsorption efficiency increased modified Bisphenol A 69.93 94.8 - AL = 0.125 g/L, [290] with the initial concentrations CNT T = 280 K, The adsorption capacity increased due to higher pores size, π–π interactions, and NH -treated 3 Chlorophenols (CP) 110.3 195 - pH 3.8, T = 298 K, hydrophobicity of [291] MWCNTs nanocomposite Effective nanomaterial with smaller equilibrium time

HNO3 and KMnO4- IC = 500 mg/L, Adsorption capacity can be Functionalized Phenol 76.92 - 88 Agitation speed = 200 greatly affected by pH and [292] MWCNTs rpm, T = 298 K adsorbent mass Open ends of nanotubes, higher IC = 0.01 mg/mL, surface area, and the functional agitation Oxidized SWNTs p-Nitrophenol (PNP) 206 - 97.9 groups (hydroxyl, carbonyl, and [293] time = 30 min carboxyl) were responsible for T = 293 K higher adsorption Sustainability 2021, 13, 5717 39 of 54

Table 9. Cont.

π–π interaction occurred Nitrogen-doped pH 7, between the functional groups carbon nanotubes Phenol 0.16 mmol/g 102 - AL = 0.6mmol/L, and phenol; [294] (CNx) 298 K, more oxidized CNTs adsorbed less phenol IC = 0.417 mg/L, Phenol 0.15 mmol/g - - AL = 10 mg, Higher adsorption of CP than MWCNT-COOH [295] T = 293 K phenol resulted because of the IC = 1.25 mg/L, different solubility of these 3-Chlorophenol (CP) 0.37 mmol/g - 95 AL = 10 mg, contaminants T = 293 K Excellent adsorbent due to Acid-functionalized pH 5.5, IC = 45 mg/L, strong interactions between MWCNT 2-Nitrophenol 256.41 197.83 - [296] T = 298 K, 2–Nitrophenol and surface (MWCNT-COOH) functional groups Q = adsorption capacity, IC = Initial concentration, AL = Adsorbent loading and T = Temperature. Sustainability 2021, 13, 5717 40 of 54

6.3. Removal of Microorganisms Bacteriological contaminants, deteriorating the assimilative capacity of water bodies leading to diverse impacts on surrounding environment, are a major challenge [297]. Bacteriological contaminants are often found in surface waters, wastewaters, and respective treatment plants [145,298]. CNTs, with their diverse range of surface and functional characteristics, have a high-affinity interaction with biological contaminants. CNTs have been proven for their higher adsorption capacities, inactivation efficiencies of viral or bacterial spores, and greater antimicrobial potential than conventional sorbents because of their greater surface area [170,299,300]. Previous studies that have reported that CNTs can inactivate or remove a variety of microorganisms, including bacteria, are shown in Table 10. The inactivation of E. coli was observed by using SWCNTs because of its penetration into the bacterial cell wall [301]. In addition, surface-modified CNTs with different chemical groups destroy the cell wall of microorganisms more strongly than the raw and polymer-grafting CNT membranes [135]. Furthermore, the straight interaction of microbes (i.e., E. coli) and functionalized CNTs can cause adverse effects on the metabolisms and morphological structure of the cell wall of bacteria [135]. The penetration of CNTs into the cell wall of microorganisms is the main reason for their higher inactivation affinity [301]. CNTs functionalized with silver nanoparticles showed excellent ability to inactivate microorganisms. For example, Ihsanullah and his coworkers [302] synthesized and suc- cessfully used silver-doped CNTs for the inactivation of E. coli bacteria. The results proved that 100% of the bacteria were killed due to toxicity of synthesized silver doped nanomate- rial [302]. In addition, many researchers reported that the diameter of nanotubes can be an important factor of concern for inactivation pathogenic microorganisms (Table 10) in water and wastewater. The single kinetics of CNTs also reveal the CNTs’ ability to eliminate pathogens in water and wastewater treatment, and microbes remain on the CNT surface based on deep filtration mechanisms [303,304]. Brady and coworkers [299] used a poly vinylidene fluoride-based SWCNT filter for excellent removal of E. coli bacteria at low pressure. The results show that the cells were completely captured by the filter [299]. In addition to filtrations, an excellent removal efficiency of MS2 virus was observed by filtering the sample through a controlled nano porous CNT based filter at a pressure of about 8 to 11 bar [304]. Beside the filtrations, CNTs are widely used in water and wastewater treatment as an antimicrobial agent, as described earlier. This behavior makes them a replacement for chemical disinfectants as a new method for controlling pathogens [301,305–308]. The applications of CNTs for disinfection treatment in water and wastewater avoid the material- ization of unsafe disinfectant by-products like trihalomethanes, aldehydes, and haloacetic acids due to their low oxidation state and solubility in water. Therefore, it is necessary to promote the dispersibility of these compounds; a surfactant or a polymer such as sodium dodecylbenzenesulfonate, polyvinylpyrrolidone is used [309]. Due to the excellent me- chanical properties on CNTs, they act as scaffolds for antimicrobial agents, such as silver nanoparticles [310,311] and antibacterial lysozyme [307,309]. Sustainability 2021, 13, 5717 41 of 54

Table 10. Functionalized CNTs used for disinfection.

Adsorbents Removal Contaminants RE Comments Ref Types AL IC Mechanism All the bacteria were inactivated Silver-doped CNT - 1 × 106 CFU/mL 100% - by membrane with 10% silver [312] membrane loadings in 60 min only Effectively inactivate the pathogen Silver-nanoparticle- from wastewater effluents, loaded 2.5 µg/mL 106 CFU/mL 89% - [313] resistance toward bacterial CNTs adhesion Physical interaction Higher antimicrobial activity at Chitosan/CNT 1.5 × 108 to 2 wt% 2.89 log reduction and surface the low contact time (10 min) and [314] Escherichia coli (E. coli) nanocomposites 5.0 × 108 CFU/mL complexation low concentration (1%) Inactivation of pathogens was due Acidic-conditioned 200 µg/mL 106 to 109 CFU/mL - Steric obstruction to both MWCNT functionalization [315] MWCNTs and nutrition level The interaction of microwaves 1-octadecanol- with f-CNTs is an innovative functionalized 0.2 g/100 mL 3.5 × 107 CFU/mL 100% Polarization approach that has the potential to [316] MWCNTS be employed for water disinfection Effectively inactivate the pathogen from wastewater effluents, CNT–Ag nanohybrid 2.5 µg/mL 106 CFU/mL 100% - [313] resistance toward bacterial adhesion Staphylococcusaureus Physical interaction Higher antimicrobial activity at Chitosan/CNTs 1.5 × 108 to 2 wt% 4.9 log reduction and surface the low contact time (10 min) and [314] nanocomposites 5.0 × 108 CFU/mL complexation low concentration (1%) Physical interaction Higher antimicrobial activity at Chitosan/CNTs 1.5 × 108 to Aspergillus flavus 2 wt% 5.5 log reduction and surface the low contact time (10 min) and [314] nanocomposites 5.0 × 108 CFU/mL complexation low concentration (1%) RE = removal efficiency (%)/log reduction; AL = adsorbent loading; IC = initial concentration, mg/L; CFU/mL = colony-forming unit per milliliter. Sustainability 2021, 13, 5717 42 of 54

7. Conclusions Purifying water from assorted contaminants is challenging, and carbon nanotube-based nanocomposites can provide simple as well as effective water decontamination/disinfection. This review shows that functionalized CNTs are a new generation of pollution management materials. These materials have excellent adsorption capacities and work effectively in remov- ing organics, inorganics, and biological species. Various sorption mechanisms include physical adsorption, electrostatic interaction, surface complexation, and chemical interactions between surface functional groups and metal ions. The effects of pH, CNT dosage, time, ionic strength, temperature, and surface charge on the adsorption of heavy metal ions on carbon nanotube surface were also discussed. Even functionalized CNTs also has antibacterial efficacy against Gram-positive and Gram-negative bacteria. Almost all the studies show effective removal of contaminants and have been per- formed using deionized water, but the potential of functionalized CNT nanomaterials needs to be verified under real water conditions. In wastewater, carbonates, phosphates, and silicates can successfully compete with target metals/organics for adsorption sites in nanos- tructures. Other important components of source water and wastewater are natural organic materials, such as fulvic acid and humic acid, which will occupy the surface of CNTs and thus affect the adsorption of contaminants on the nanostructures. The effectiveness of CNT-based nanotechnology should be evaluated under real water conditions. Many researchers have also focused on evaluating the adsorption–desorption capacity to make it a cost-effective adsorbent for use in wastewater treatment. However, more studies are encouraged to check the feasibility of reuse.

Author Contributions: Conceptualization, W.D. and M.M.-A.A.; resources, W.D. and H.-W.K.; writing—original draft preparation, M.M.-A.A.; writing—review and editing, H.-W.K., W.D., M.S., M.U., M.M.-A.A.; visualization, H.A.; supervision, H.-W.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article. Acknowledgments: This study was done at the Department of Environmental Science and Engineer- ing, Tunghai University, Taiwan. The authors gratefully acknowledge the funding support by the Research Council of Texas A&M University San Antonio, and the Ministry of Science and Technology of R.O.C. (Taiwan), 107-2221-E-029-001-MY2. The authors would also like to thank Yu-Ling Wei, Chiung-Fen Chang, and Meng-Hau Sung for providing financial and analytical support to M.M.A.A. We acknowledge support of the Hamburg University of Technology (TUHH) by enabling open access publishing through funding programme Open Access Publishing. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Ali, I.; Li, J.; Peng, C.; Qasim, M.; Khan, Z.M.; Naz, I.; Sultan, M.; Rauf, M.; Iqbal, W.; Sharif, H.M.A. 3-Dimensional membrane capsules: Synthesis modulations for the remediation of environmental pollutants—A critical review. Crit. Rev. Environ. Sci. Technol. 2020, 1–62. [CrossRef] 2. Usman, M.; Waseem, M.; Mani, N.; Andiego, N. Optimization of soil aquifer treatment by chemical oxidation with hydrogen peroxide addition. Pollution 2018, 4, 369–379. [CrossRef] 3. Usman, M. New Applications of Fine-Grained Iron Oxyhydroxides as Cost-Effective Arsenic Adsorbents in Water Treatment. Ph.D. Thesis, Technische Universität Hamburg, Hamburg, Germany, 2020. [CrossRef] 4. Islam, T.; Peng, C.; Ali, I.; Li, J.; Khan, Z.M.; Sultan, M.; Naz, I. Synthesis of rice husk-derived magnetic biochar through Liquefaction to adsorb anionic and cationic dyes from aqueous solutions. Arab. J. Sci. Eng. 2021, 46, 233–246. [CrossRef] 5. Khan, S.U.; Farooqi, I.H.; Usman, M.; Basheer, F. Energy efficient rapid removal of arsenic in an electrocoagulation reactor with hybrid Fe/Al electrodes: Process optimization using CCD and kinetic modeling. Water 2020, 12, 2876. [CrossRef] Sustainability 2021, 13, 5717 43 of 54

6. Usman, M.; Katsoyiannis, I.; Rodrigues, J.H.; Ernst, M. Arsenate removal from drinking water using by-products from conven- tional iron oxyhydroxides production as adsorbents coupled with submerged microfiltration unit. Environ. Sci. Pollut. Res. 2020. [CrossRef][PubMed] 7. Amjed, M.A.; Peng, C.; Dai, M.; Chang, Q.; Ali, I.; Sultan, M.; Naz, I.; Farooq, M.Z.; Kashif, M. Recent updates on the solar-assisted biochar production and potential usage for water treatment. Fresenius Environ. Bull. 2020, 29, 5616–5632. 8. Cheng, N.; Wang, B.; Wu, P.; Lee, X.; Xing, Y.; Chen, M.; Gao, B. Adsorption of emerging contaminants from water and wastewater by modified biochar: A review. Environ. Pollut. 2021, 273, 116448. [CrossRef][PubMed] 9. Aslam, M.M.A.; Khan, Z.M.; Sultan, M.; Niaz, Y.; Mahmood, M.H.; Shoaib, M.; Shakoor, A.; Ahmad, M. Performance evaluation of trickling filter-based wastewater treatment system utilizing cotton sticks as filter media. Polish J. Environ. Stud. 2017, 26, 1955–1962. [CrossRef] 10. Khan, Z.M.; Kanwar, R.M.A.; Farid, H.U.; Sultan, M.; Arsalan, M.; Ahmad, M.; Shakoor, A.; Aslam, M.M.A. Wastewater evaluation for multan, pakistan: Characterization and agricultural reuse. Polish J. Environ. Stud. 2019, 28, 2159–2174. [CrossRef] 11. Aslam, M.M.A.; Den, W.; Kuo, H.W. Encapsulated chitosan-modified magnetic carbon nanotubes for aqueous-phase CrVI uptake. J. Water Process Eng. 2021, 40, 101793. [CrossRef] 12. Den, W.; Wang, C.J. Removal of silica from brackish water by electrocoagulation pretreatment to prevent fouling of reverse osmosis membranes. Sep. Purif. Technol. 2008, 59, 318–325. [CrossRef] 13. Su, Y.N.; Lin, W.S.; Hou, C.H.; Den, W. Performance of integrated membrane filtration and electrodialysis processes for copper recovery from wafer polishing wastewater. J. Water Process Eng. 2014, 4, 149–158. [CrossRef] 14. Ali, I.; Peng, C.; Khan, Z.M.; Naz, I.; Sultan, M.; Ali, M.; Abbasi, I.A.; Islam, T.; Ye, T. Overview of microbes based fabricated biogenic nanoparticles for water and wastewater treatment. J. Environ. Manag. 2019, 230, 128–150. [CrossRef] 15. Ali, I.; Peng, C.; Khan, Z.M.; Naz, I.; Sultan, M. An overview of heavy metal removal from wastewater using magnetotactic bacteria. J. Chem. Technol. Biotechnol. 2018, 93, 2817–2832. [CrossRef] 16. Gupta, V.K.; Agarwal, S.; Saleh, T.A. Chromium removal by combining the magnetic properties of iron oxide with adsorption properties of carbon nanotubes. Water Res. 2011, 45, 2207–2212. [CrossRef] 17. Usman, M.; Belkasmi, A.I.; Katsoyiannis, I.A.; Ernst, M. Pre-deposited dynamic membrane adsorber formed of microscale conventional iron oxide-based adsorbents to remove arsenic from water: Application study and mathematical modeling. J. Chem. Technol. Biotechnol. 2021.[CrossRef] 18. Chai, W.S.; Cheun, J.Y.; Kumar, P.S.; Mubashir, M.; Majeed, Z.; Banat, F.; Ho, S.-H.; Show, P.L. A review on conventional and novel materials towards heavy metal adsorption in wastewater treatment application. J. Clean. Prod. 2021, 296, 126589. [CrossRef] 19. Usman, M.; Zarebanadkouki, M.; Waseem, M.; Katsoyiannis, I.A.; Ernst, M. Mathematical modeling of arsenic(V) adsorption onto iron oxyhydroxides in an adsorption-submerged membrane hybrid system. J. Hazard. Mater. 2020, 400, 123221. [CrossRef] [PubMed] 20. Kyzas, G.Z.; Matis, K.A. Nanoadsorbents for pollutants removal: A review. J. Mol. Liq. 2015, 203, 159–168. [CrossRef] 21. Trujillo-Reyes, J.; Peralta-Videa, J.R.; Gardea-Torresdey, J.L. Supported and unsupported nanomaterials for water and soil remediation: Are they a useful solution for worldwide pollution? J. Hazard. Mater. 2014, 280, 487–503. [CrossRef] 22. Sharma, V.K.; McDonald, T.J.; Kim, H.; Garg, V.K. Magnetic graphene–carbon nanotube iron nanocomposites as adsorbents and antibacterial agents for water purification. Adv. Colloid Interface Sci. 2015, 225, 229–240. [CrossRef][PubMed] 23. Qu, X.; Alvarez, P.J.J.; Li, Q. Applications of nanotechnology in water and wastewater treatment. Water Res. 2013, 47, 3931–3946. [CrossRef][PubMed] 24. Usman, M.; Katsoyiannis, I.; Mitrakas, M.; Zouboulis, A.; Ernst, M. Performance evaluation of small sized powdered ferric hydroxide as arsenic adsorbent. Water 2018, 10, 957. [CrossRef] 25. Goh, K.; Karahan, H.E.; Wei, L.; Bae, T.-H.; Fane, A.G.; Wang, R.; Chen, Y. Carbon nanomaterials for advancing separation membranes: A strategic perspective. Carbon N. Y. 2016, 109, 694–710. [CrossRef] 26. Goh, P.S.; Ismail, A.F.; Hilal, N. Nano-enabled membranes technology: Sustainable and revolutionary solutions for membrane desalination? Desalination 2016, 380, 100–104. [CrossRef] 27. Goh, P.S.; Matsuura, T.; Ismail, A.F.; Hilal, N. Recent trends in membranes and membrane processes for desalination. Desalination 2016, 391, 43–60. [CrossRef] 28. Gupta, V.K.; Agarwal, S.; Saleh, T.A. Synthesis and characterization of alumina-coated carbon nanotubes and their application for lead removal. J. Hazard. Mater. 2011, 185, 17–23. [CrossRef] 29. Santhosh, C.; Velmurugan, V.; Jacob, G.; Jeong, S.K.; Grace, A.N.; Bhatnagar, A. Role of nanomaterials in water treatment applications: A review. Chem. Eng. J. 2016, 306, 1116–1137. [CrossRef] 30. Xu, P.; Zeng, G.M.; Huang, D.L.; Feng, C.L.; Hu, S.; Zhao, M.H.; Lai, C.; Wei, Z.; Huang, C.; Xie, G.X.; et al. Use of iron oxide nanomaterials in wastewater treatment: A review. Sci. Total Environ. 2012, 424, 1–10. [CrossRef] 31. Pendergast, M.M.; Hoek, E.M.V. A review of water treatment membrane nanotechnologies. Energy Environ. Sci. 2011, 4, 1946. [CrossRef] 32. Bethi, B.; Sonawane, S.H.; Bhanvase, B.A.; Gumfekar, S.P. Nanomaterials-based advanced oxidation processes for wastewater treatment: A review. Chem. Eng. Process. Process Intensif. 2016, 109, 178–189. [CrossRef] 33. Holmes, A.B.; Gu, F.X. Emerging nanomaterials for the application of selenium removal for wastewater treatment. Environ. Sci. Nano 2016, 3, 982–996. [CrossRef] Sustainability 2021, 13, 5717 44 of 54

34. Lee, J.; Jeong, S.; Liu, Z. Progress and challenges of carbon nanotube membrane in water treatment. Crit. Rev. Environ. Sci. Technol. 2016, 46, 999–1046. [CrossRef] 35. Olivera, S.; Muralidhara, H.B.; Venkatesh, K.; Guna, V.K.; Gopalakrishna, K.; Kumar, K.Y. Potential applications of cellulose and chitosan nanoparticles/composites in wastewater treatment: A review. Carbohydr. Polym. 2016, 153, 600–618. [CrossRef] 36. Ong, C.S.; Goh, P.S.; Lau, W.J.; Misdan, N.; Ismail, A.F. Nanomaterials for biofouling and scaling mitigation of thin film composite membrane: A review. Desalination 2016, 393, 2–15. [CrossRef] 37. Stefaniuk, M.; Oleszczuk, P.; Ok, Y.S. Review on nano zerovalent iron (nZVI): From synthesis to environmental applications. Chem. Eng. J. 2016, 287, 618–632. [CrossRef] 38. Monthioux, M.; Kuznetsov, V.L. Who should be given the credit for the discovery of carbon nanotubes? Carbon N. Y. 2006, 44, 1621–1623. [CrossRef] 39. Radushkevich, L.V.; Lukyanovich, V.M. About the structure of carbon formed by thermal decomposition of carbon monoxide on iron substrate. J. Phys. Chem. 1952, 26, 88–95. 40. Oberlin, A.; Endo, M.; Koyama, T. Filamentous growth of carbon through benzene decomposition. J. Cryst. Growth 1976, 32, 335–349. [CrossRef] 41. Abrahamson, J.; Wiles, P.G.; Rhoades, B.L. Structure of carbon fibres found on carbon arc anodes. Carbon N. Y. 1999, 37, 1873–1874. [CrossRef] 42. Hirlekar, R.; Yamagar, M.; Garse, H.; Vij, M.; Kadam, V.; Vidyapeeth, B. Carbon nanotubes and its applications: A review. Asian J. Pharm. Clin. Res. 2009, 2, 17–27. 43. Tennent, H.G.; Barber, J.J.; Hoch, R. Carbon Fibrils, Method for Producing Same and Compositions Containing Same. U.S. Patent 4,663,230, 5 May 1987. 44. Bethune, D.S.; Kiang, C.H.; de Vries, M.S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls. Nature 1993, 363, 605–607. [CrossRef] 45. Iijima, S.; Ichihashi, T. Single-shell carbon nanotubes of 1-nm diameter. Nature 1993, 363, 603–605. [CrossRef] 46. Krätschmer, W.; Lamb, L.D.; Fostiropoulos, K.; Huffman, D.R. Solid C60: A new form of carbon. Nature 1990, 347, 354–358. [CrossRef] 47. Kroto, H.W.; Heath, J.R.; O’Brien, S.C.; Curl, R.F.; Smalley, R.E. C60: Buckminsterfullerene. Nature 1985, 318, 162–163. [CrossRef] 48. Liu, Z.; Sun, X.; Nakayama-Ratchford, N.; Dai, H. Supramolecular Chemistry on Water-Soluble Carbon Nanotubes for Drug Loading and Delivery. ACS Nano 2007, 1, 50–56. [CrossRef] 49. Singh, B.G.P.; Baburao, C.; Pispati, V.; Pathipati, H.; Muthy, N.; Prassana, S.R.V.; Rathode, B.G. Carbon nanotubes. A novel drug delivery system. Int. J. Res. Pharm. Chem. 2012, 2, 523–532. 50. Lam, C.; James, J.T.; McCluskey, R.; Arepalli, S.; Hunter, R.L. A review of carbon nanotube toxicity and assessment of potential occupational and environmental health risks. Crit. Rev. Toxicol. 2006, 36, 189–217. [CrossRef][PubMed] 51. Bekyarova, E.; Ni, Y.; Malarkey, E.B.; Montana, V.; McWilliams, J.L.; Haddon, R.C.; Parpura, V. Applications of Carbon Nanotubes in Biotechnology and Biomedicine. J. Biomed. Nanotechnol. 2005, 1, 3–17. [CrossRef] 52. He, H.; Pham-Huy, L.A.; Dramou, P.; Xiao, D.; Zuo, P.; Pham-Huy, C. Carbon Nanotubes: Applications in Pharmacy and Medicine. Biomed. Res. Int. 2013, 2013, 1–12. [CrossRef] 53. Reilly, R.M. Carbon Nanotubes: Potential Benefits and Risks of Nanotechnology in Nuclear Medicine. J. Nucl. Med. 2007, 48, 1039–1042. [CrossRef][PubMed] 54. Xie, X.; Mai, Y.; Zhou, X. Dispersion and alignment of carbon nanotubes in polymer matrix: A review. Mater. Sci. Eng. R Rep. 2005, 49, 89–112. [CrossRef] 55. Ihsanullah, A.A.; Al-Amer, A.M.; Laoui, T.; Al-Marri, M.J.; Nasser, M.S.; Khraisheh, M.; Atieh, M.A. Heavy metal removal from aqueous solution by advanced carbon nanotubes: Critical review of adsorption applications. Sep. Purif. Technol. 2016, 157, 141–161. [CrossRef] 56. Balasubramanian, K.; Burghard, M. Chemically Functionalized Carbon Nanotubes. Small 2005, 1, 180–192. [CrossRef][PubMed] 57. Monea, B.F.; Ionete, E.I.; Spiridon, S.I.; Ion-Ebrasu, D.; Petre, E. Carbon Nanotubes and Carbon Nanotube Structures Used for Temperature Measurement. Sensors 2019, 19, 2464. [CrossRef] 58. Digge, M.; Moon, R.; Gattani, S. Application of Carbon Nanotubes in Drug Delivery: A Review. Int. J. PharmTech Res. 2011, 4, 839–847. 59. Kateb, B.; Yamamoto, V.; Alizadeh, D.; Zhang, L.; Manohara, H.M.; Bronikowski, M.J.; Badie, B. Multi-walled Carbon Nanotube (MWCNT) Synthesis, Preparation, Labeling, and Functionalization. In Immunotherapy of Cancer; Humana Press: Totowa, NJ, USA, 2010; pp. 307–317. 60. Liao, H.; Paratala, B.; Sitharaman, B.; Wang, Y. Applications of Carbon Nanotubes in Biomedical Studies. In Biomedical Nanotechnology; Humana Press: Totowa, NJ, USA, 2011; pp. 223–241. 61. Usui, Y.; Haniu, H.; Tsuruoka, S.; Saito, N. Carbon nanotubes innovate on medical technology. Med. Chem 2012, 2, 1–6. [CrossRef] 62. Yang, D.; Yang, F.; Hu, J.; Long, J.; Wang, C.; Fu, D.; Ni, Q. Hydrophilic multi-walled carbon nanotubes decorated with magnetite nanoparticles as lymphatic targeted drug delivery vehicles. Chem. Commun. 2009, 4447. [CrossRef] 63. Zhang, Y.; Bai, Y.; Yan, B. Functionalized carbon nanotubes for potential medicinal applications. Drug Discov. Today 2010, 15, 428–435. [CrossRef] Sustainability 2021, 13, 5717 45 of 54

64. Cassell, A.M.; Raymakers, J.A.; Kong, J.; Dai, H. Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes. J. Phys. Chem. B 1999, 103, 6484–6492. [CrossRef] 65. Sinnott, S.B.; Andrews, R.; Qian, D.; Rao, A.M.; Mao, Z.; Dickey, E.C.; Derbyshire, F. Model of carbon nanotube growth through chemical vapor deposition. Chem. Phys. Lett. 1999, 315, 25–30. [CrossRef] 66. Vinciguerra, V.; Buonocore, F.; Panzera, G.; Occhipinti, L. Growth mechanisms in chemical vapour deposited carbon nanotubes. Nanotechnology 2003, 14, 655. [CrossRef] 67. Helveg, S.; López-Cartes, C.; Sehested, J.; Hansen, P.L.; Clausen, B.S.; Rostrup-Nielsen, J.R.; Abild-Pedersen, F.; Nørskov, J.K. Atomic-scale imaging of carbon nanofibre growth. Nature 2004, 427, 426–429. [CrossRef][PubMed] 68. Maser, W.K.; Benito, A.M.; Martınez, M.T. Production of carbon nanotubes: The light approach. Carbon N. Y. 2002, 40, 1685–1695. [CrossRef] 69. Kingston, C.T.; Simard, B. Fabrication of Carbon Nanotubes. Anal. Lett. 2003, 36, 3119–3145. [CrossRef] 70. Hutchison, J.L.; Kiselev, N.A.; Krinichnaya, E.P.; Krestinin, A.V.; Loutfy, R.O.; Morawsky, A.P.; Muradyan, V.E.; Obraztsova, E.D.; Sloan, J.; Terekhov, S.V.; et al. Double-walled carbon nanotubes fabricated by a hydrogen arc discharge method. Carbon N. Y. 2001, 39, 761–770. [CrossRef] 71. Shi, Z.; Lian, Y.; Zhou, X.; Gu, Z.; Zhang, Y.; Iijima, S.; Zhou, L.; Yue, K.T.; Zhang, S. Mass-production of single-wall carbon nanotubes by arc discharge method11This work was supported by the National Natural Science Foundation of China, No. 29671030. Carbon N. Y. 1999, 37, 1449–1453. [CrossRef] 72. Sano, N.; Wang, H.; Chhowalla, M.; Alexandrou, I.; Amaratunga, G.A.J. Synthesis of carbon “onions” in water. Nature 2001, 414, 506–507. [CrossRef] 73. Li, H.; Guan, L.; Shi, Z.; Gu, Z. Direct Synthesis of High Purity Single-Walled Carbon Nanotube Fibers by Arc Discharge. J. Phys. Chem. B 2004, 108, 4573–4575. [CrossRef] 74. Imasaka, K.; Kanatake, Y.; Ohshiro, Y.; Suehiro, J.; Hara, M. Production of carbon nanoonions and nanotubes using an intermittent arc discharge in water. Thin Solid Films 2006, 506–507, 250–254. [CrossRef] 75. Sagara, T.; Kurumi, S.; Suzuki, K. Growth of linear Ni-filled carbon nanotubes by local arc discharge in liquid ethanol. Appl. Surf. Sci. 2014, 292, 39–43. [CrossRef] 76. Ben Belgacem, A.; Hinkov, I.; Yahia, S.B.; Brinza, O.; Farhat, S. Arc discharge boron nitrogen doping of carbon nanotubes. Mater. Today Commun. 2016, 8, 183–195. [CrossRef] 77. Berkmans, J.; Jagannatham, M.; Reddy, R.; Haridoss, P. Synthesis of thin bundled single walled carbon nanotubes and nanohorn hybrids by arc discharge technique in open air atmosphere. Diam. Relat. Mater. 2015, 55, 12–15. [CrossRef] 78. Su, Y.; Zhang, Y. Carbon nanomaterials synthesized by arc discharge hot plasma. Carbon N. Y. 2015, 83, 90–99. [CrossRef] 79. Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D.T.; Smalley, R.E. Catalytic growth of single-walled manotubes by laser vaporization. Chem. Phys. Lett. 1995, 243, 49–54. [CrossRef] 80. Guo, T.; Nikolaev, P.; Rinzler, A.G.; Tomanek, D.; Colbert, D.T.; Smalley, R.E. Self-assembly of tubular fullerenes. J. Phys. Chem. 1995, 99, 10694–10697. [CrossRef] 81. Nagy, J.B.; Bister, G.; Fonseca, A.; Méhn, D.; Kónya, Z.; Kiricsi, I.; Horváth, Z.E.; Biró, L.P. On the Growth Mechanism of Single-Walled Carbon Nanotubes by Catalytic Carbon Vapor Deposition on Supported Metal Catalysts. J. Nanosci. Nanotechnol. 2004, 4, 326–345. [CrossRef] 82. Bandaru, P.R. Electrical Properties and Applications of Carbon Nanotube Structures. J. Nanosci. Nanotechnol. 2007, 7, 1239–1267. [CrossRef] 83. Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y.H.; Kim, S.G.; Rinzler, A.G.; et al. Crystalline Ropes of Metallic Carbon Nanotubes. Science 1996, 273, 483–487. [CrossRef] 84. Journet, C.; Bernier, P. Production of carbon nanotubes. Appl. Phys. A Mater. Sci. Process. 1998, 67, 1–9. [CrossRef] 85. Ebbesen, T.W.; Hiura, H.; Fujita, J.; Ochiai, Y.; Matsui, S.; Tanigaki, K. Patterns in the bulk growth of carbon nanotubes. Chem. Phys. Lett. 1993, 209, 83–90. [CrossRef] 86. José-Yacamán, M.; Miki-Yoshida, M.; Rendón, L.; Santiesteban, J.G. Catalytic growth of carbon microtubules with fullerene structure. Appl. Phys. Lett. 1993, 62, 202–204. [CrossRef] 87. Ren, Z.F. Synthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glass. Science 1998, 282, 1105–1107. [CrossRef] 88. Oliver, J. Global Markets and Technologies for Carbon Nanotubes: NAN024F BCC Research; BCC Publishing: Wellesley, MA, USA, 2015. 89. Kumar, S.; Rani, R.; Dilbaghi, N.; Tankeshwar, K.; Kim, K.-H. Carbon nanotubes: A novel material for multifaceted applications in human healthcare. Chem. Soc. Rev. 2017, 46, 158–196. [CrossRef][PubMed] 90. Ferreira, F.V.; Franceschi, W.; Menezes, B.R.C.; Biagioni, A.F.; Coutinho, A.R.; Cividanes, L.S. Synthesis, Characterization, and Applications of Carbon Nanotubes. In Carbon-Based Nanofillers and Their Rubber Nanocomposites; Elsevier: Amsterdam, The Netherlands, 2019; pp. 1–45. 91. Donaldson, K.; Aitken, R.; Tran, L.; Stone, V.; Duffin, R.; Forrest, G.; Alexander, A. Carbon Nanotubes: A Review of Their Properties in Relation to Pulmonary Toxicology and Workplace Safety. Toxicol. Sci. 2006, 92, 5–22. [CrossRef] 92. Ando, Y.; Zhao, X.; Sugai, T.; Kumar, M. Growing carbon nanotubes. Mater. Today 2004, 7, 22–29. [CrossRef] Sustainability 2021, 13, 5717 46 of 54

93. Muataz, A.A.; Fakhrul-Razi, A.; Dayang, B.A.R.; El-Sadig, M.; Chuah, G.T.; Maan, F.A.; Sunny, I.; Faizah, Y.; Abdul Hamid, M.; Halim, M. Production of vapor growth carbon fiber (vgcf) by using cvd. In Proceedings of the 17th Symposium of Malaysian Chemical Engineers (SOMChE 2003) “Role of Chemical Engineers for Sustainability of Small Medium Industries (SMI)”, Penang, Malaysia, 29–30 December 2003; pp. 596–602. 94. Cao, Z.; Sun, Z.; Guo, P.; Chen, Y. Effect of acetylene flow rate on morphology and structure of carbon nanotube thick films grown by thermal chemical vapor deposition. Front. Mater. Sci. China 2007, 1, 92–96. [CrossRef] 95. Fonseca, A.; Hernadi, K.; Piedigrosso, P.; Colomer, J.-F.; Mukhopadhyay, K.; Doome, R.; Lazarescu, S.; Biro, L.P.; Lambin, P.; Thiry, P.A.; et al. Synthesis of single- and multi-wall carbon nanotubes over supported catalysts. Appl. Phys. A Mater. Sci. Process. 1998, 67, 11–22. [CrossRef] 96. Zheng, L.X.; O’Connell, M.J.; Doorn, S.K.; Liao, X.Z.; Zhao, Y.H.; Akhadov, E.A.; Hoffbauer, M.A.; Roop, B.J.; Jia, Q.X.; Dye, R.C.; et al. Ultralong single-wall carbon nanotubes. Nat. Mater. 2004, 3, 673–676. [CrossRef] 97. Kim, N.S.; Lee, Y.T.; Park, J.; Han, J.B.; Choi, Y.S.; Choi, S.Y.; Choo, J.; Lee, G.H. Vertically Aligned Carbon Nanotubes Grown by Pyrolysis of Iron, Cobalt, and Nickel Phthalocyanines. J. Phys. Chem. B 2003, 107, 9249–9255. [CrossRef] 98. Bustero, I.; Ainara, G.; Isabel, O.; Roberto, M.; Inés, R.; Amaya, A. Control of the Properties of Carbon Nanotubes Synthesized by CVD for Application in Electrochemical Biosensors. Microchim. Acta 2006, 152, 239–247. [CrossRef] 99. Mohamed, M.M.; Ghanem, M.A.; Khairy, M.; Naguib, E.; Alotaibi, N.H. Zinc oxide incorporated carbon nanotubes or graphene oxide nanohybrids for enhanced sonophotocatalytic degradation of methylene blue dye. Appl. Surf. Sci. 2019, 487, 539–549. [CrossRef] 100. Kong, J.; Cassell, A.M.; Dai, H. Chemical vapor deposition of methane for single-walled carbon nanotubes. Chem. Phys. Lett. 1998, 292, 567–574. [CrossRef] 101. Pérez-Cabero, M.; Monzón, A.; Rodrıguez-Ramos, I.; Guerrero-Ruız, A. Syntheses of CNTs over several iron-supported catalysts: Influence of the metallic precursors. Catal. Today 2004, 93–95, 681–687. [CrossRef] 102. Zhu, S.; Su, C.-H.; Lehoczky, S.L.; Muntele, I.; Ila, D. Carbon nanotube growth on carbon fibers. Diam. Relat. Mater. 2003, 12, 1825–1828. [CrossRef] 103. Zhu, W.Z.; Miser, D.E.; Chan, W.G.; Hajaligol, M.R. Characterization of multiwalled carbon nanotubes prepared by carbon arc cathode deposit. Mater. Chem. Phys. 2003, 82, 638–647. [CrossRef] 104. Collins, P.G.; Avouris, P. Nanotubes for electronics. Sci. Am. 2000, 283, 62–69. [CrossRef] 105. Colomer, J.-F.; Stephan, C.; Lefrant, S.; Van Tendeloo, G.; Willems, I.; Kónya, Z.; Fonseca, A.; Laurent, C.; Nagy, J. Large-scale synthesis of single-wall carbon nanotubes by catalytic chemical vapor deposition (CCVD) method. Chem. Phys. Lett. 2000, 317, 83–89. [CrossRef] 106. Ebbesen, T.W.; Ajayan, P.M. Large-scale synthesis of carbon nanotubes. Nature 1992, 358, 220–222. [CrossRef] 107. Ren, Z.F.; Huang, Z.P.; Wang, D.Z.; Wen, J.G.; Xu, J.W.; Wang, J.H.; Calvet, L.E.; Chen, J.; Klemic, J.F.; Reed, M.A. Growth of a single freestanding multiwall carbon nanotube on each nanonickel dot. Appl. Phys. Lett. 1999, 75, 1086–1088. [CrossRef] 108. Yudasaka, M.; Kikuchi, R.; Matsui, T.; Ohki, Y.; Yoshimura, S.; Ota, E. Specific conditions for Ni catalyzed carbon nanotube growth by chemical vapor deposition. Appl. Phys. Lett. 1995, 67, 2477–2479. [CrossRef] 109. Yudasaka, M.; Kikuchi, R.; Ohki, Y.; Ota, E.; Yoshimura, S. Behavior of Ni in carbon nanotube nucleation. Appl. Phys. Lett. 1997, 70, 1817–1818. [CrossRef] 110. Eklund, P.C.; Pradhan, B.K.; Kim, U.J.; Xiong, Q.; Fischer, J.E.; Friedman, A.D.; Holloway, B.C.; Jordan, K.; Smith, M.W. Large-Scale Production of Single-Walled Carbon Nanotubes Using Ultrafast Pulses from a Free Electron Laser. Nano Lett. 2002, 2, 561–566. [CrossRef] 111. Maser, W.K.; Muñoz, E.; Benito, A.M.; Martınez, M.T.; de la Fuente, G.F.; Maniette, Y.; Anglaret, E.; Sauvajol, J.-L. Production of high-density single-walled nanotube material by a simple laser-ablation method. Chem. Phys. Lett. 1998, 292, 587–593. [CrossRef] 112. Bolshakov, A.P.; Uglov, S.A.; Saveliev, A.V.; Konov, V.I.; Gorbunov, A.A.; Pompe, W.; Graff, A. A novel CW laser–powder method of carbon single-wall nanotubes production. Diam. Relat. Mater. 2002, 11, 927–930. [CrossRef] 113. Scott, C.D.; Arepalli, S.; Nikolaev, P.; Smalley, R.E. Growth mechanisms for single-wall carbon nanotubes in a laser-ablation process. Appl. Phys. A Mater. Sci. Process. 2001, 72, 573–580. [CrossRef] 114. Aqel, A.; El-Nour, K.M.M.A.; Ammar, R.A.A.; Al-Warthan, A. Carbon nanotubes, science and technology part (I) structure, synthesis and characterisation. Arab. J. Chem. 2012, 5, 1–23. [CrossRef] 115. Gong, K.; Ci, L. Process for purification of carbon nanotubes. Carbon 2008, 46, 2003–2025. 116. Hou, P.; Bai, S.; Yang, Q.; Liu, C.; Cheng, H. Multi-step purification of carbon nanotubes. Carbon N. Y. 2002, 40, 81–85. [CrossRef] 117. Hajime, G.; Terumi, F.; Yoshiya, F.; Toshiyuki, O. Method of Purifying Single Wall Carbon Nanotubes from Metal Catalyst Impurities; JP Patent Filed & Issued; Honda Giken Kogyo Kabushiki Kaisha: Tokyo, Japan, 2002. 118. Borowiak-Palen, E.; Pichler, T.; Liu, X.; Knupfer, M.; Graff, A.; Jost, O.; Pompe, W.; Kalenczuk, R.; Fink, J. Reduced diameter distribution of single-wall carbon nanotubes by selective oxidation. Chem. Phys. Lett. 2002, 363, 567–572. [CrossRef] 119. Huang, S.; Dai, L. Plasma Etching for Purification and Controlled Opening of Aligned Carbon Nanotubes. J. Phys. Chem. B 2002, 106, 3543–3545. [CrossRef] 120. Chiang, Y.-C.; Chen, C.-H.; Chiang, Y.-C.; Chen, S.-L. Circulating inclined fluidized beds with application for desiccant dehumidi- fication systems. Appl. Energy 2016, 175, 199–211. [CrossRef] Sustainability 2021, 13, 5717 47 of 54

121. Harutyunyan, A.R.; Pradhan, B.K.; Chang, J.; Chen, G.; Eklund, P.C. Purification of Single-Wall Carbon Nanotubes by Selective Microwave Heating of Catalyst Particles. J. Phys. Chem. B 2002, 106, 8671–8675. [CrossRef] 122. Farkas, E.; Elizabeth Anderson, M.; Chen, Z.; Rinzler, A.G. Length sorting cut single wall carbon nanotubes by high performance liquid chromatography. Chem. Phys. Lett. 2002, 363, 111–116. [CrossRef] 123. Chiang, I.W.; Brinson, B.E.; Huang, A.Y.; Willis, P.A.; Bronikowski, M.J.; Margrave, J.L.; Smalley, R.E.; Hauge, R.H. Purification and Characterization of Single-Wall Carbon Nanotubes (SWNTs) Obtained from the Gas-Phase Decomposition of CO (HiPco Process). J. Phys. Chem. B 2001, 105, 8297–8301. [CrossRef] 124. Chiang, I.W.; Brinson, B.E.; Smalley, R.E.; Margrave, J.L.; Hauge, R.H. Purification and Characterization of Single-Wall Carbon Nanotubes. J. Phys. Chem. B 2001, 105, 1157–1161. [CrossRef] 125. Kajiura, H.; Tsutsui, S.; Huang, H.; Murakami, Y. High-quality single-walled carbon nanotubes from arc-produced soot. Chem. Phys. Lett. 2002, 364, 586–592. [CrossRef] 126. Moon, J.-M.; An, K.H.; Lee, Y.H.; Park, Y.S.; Bae, D.J.; Park, G.-S. High-Yield Purification Process of Singlewalled Carbon Nanotubes. J. Phys. Chem. B 2001, 105, 5677–5681. [CrossRef] 127. Doi, M.; Ikuga, Y.; Kimura, T.; Mitsuzuka, H. Laminated Sheet and Diaper and Article for Sanitary Use. JP Patent JPH10713A, Application JP8116796A, 10 May 1996. 128. Bandow, S.; Rao, A.M.; Williams, K.A.; Thess, A.; Smalley, R.E.; Eklund, P.C. Purification of Single-Wall Carbon Nanotubes by Microfiltration. J. Phys. Chem. B 1997, 101, 8839–8842. [CrossRef] 129. Sajid, M.I.; Jamshaid, U.; Jamshaid, T.; Zafar, N.; Fessi, H.; Elaissari, A. Carbon nanotubes from synthesis to in vivo biomedical applications. Int. J. Pharm. 2016, 501, 278–299. [CrossRef] 130. Hou, P.-X.; Liu, C.; Cheng, H.-M. Purification of carbon nanotubes. Carbon N. Y. 2008, 46, 2003–2025. [CrossRef] 131. Sato, Y.; Ogawa, T.; Motomiya, K.; Shinoda, K.; Jeyadevan, B.; Tohji, K.; Kasuya, A.; Nishina, Y. Purification of MWNTs Combining Wet Grinding, Hydrothermal Treatment, and Oxidation. J. Phys. Chem. B 2001, 105, 3387–3392. [CrossRef] 132. Das, R.; Abd Hamid, S.B.; Ali, M.E.; Ismail, A.F.; Annuar, M.S.M.; Ramakrishna, S. Multifunctional carbon nanotubes in water treatment: The present, past and future. Desalination 2014, 354, 160–179. [CrossRef] 133. Long, R.Q.; Yang, R.T. Carbon Nanotubes as Superior Sorbent for Dioxin Removal. J. Am. Chem. Soc. 2001, 123, 2058–2059. [CrossRef] 134. Jun, L.Y.; Mubarak, N.M.; Yee, M.J.; Yon, L.S.; Bing, C.H.; Khalid, M.; Abdullah, E.C. An overview of functionalised carbon nanomaterial for organic pollutant removal. J. Ind. Eng. Chem. 2018, 67, 175–186. [CrossRef] 135. Ali, S.; Rehman, S.A.U.; Luan, H.-Y.; Farid, M.U.; Huang, H. Challenges and opportunities in functional carbon nanotubes for membrane-based water treatment and desalination. Sci. Total Environ. 2019, 646, 1126–1139. [CrossRef] 136. Ihsanullah; Al Amer, A.M.; Laoui, T.; Abbas, A.; Al-Aqeeli, N.; Patel, F.; Khraisheh, M.; Atieh, M.A.; Hilal, N. Fabrication and antifouling behaviour of a carbon nanotube membrane. Mater. Des. 2016, 89, 549–558. [CrossRef] 137. Ihsanullah; Al-Khaldi, F.A.; Abu-Sharkh, B.; Abulkibash, A.M.; Qureshi, M.I.; Laoui, T.; Atieh, M.A. Effect of acid modification on adsorption of hexavalent chromium (Cr(VI)) from aqueous solution by activated carbon and carbon nanotubes. Desalin. Water Treat. 2016, 57, 7232–7244. [CrossRef] 138. Garzia Trulli, M.; Sardella, E.; Palumbo, F.; Palazzo, G.; Giannossa, L.C.; Mangone, A.; Comparelli, R.; Musso, S.; Favia, P. Towards highly stable aqueous dispersions of multi-walled carbon nanotubes: The effect of oxygen plasma functionalization. J. Colloid Interface Sci. 2017, 491, 255–264. [CrossRef][PubMed] 139. Georgakilas, V.; Bourlinos, A.; Gournis, D.; Tsoufis, T.; Trapalis, C.; Mateo-Alonso, A.; Prato, M. Multipurpose Organically Modified Carbon Nanotubes: From Functionalization to Nanotube Composites. J. Am. Chem. Soc. 2008, 130, 8733–8740. [CrossRef] [PubMed] 140. Bounos, G.; Andrikopoulos, K.S.; Moschopoulou, H.; Lainioti, G.C.; Roilo, D.; Checchetto, R.; Ioannides, T.; Kallitsis, J.K.; Voyiatzis, G.A. Enhancing water vapor permeability in mixed matrix polypropylene membranes through carbon nanotubes dispersion. J. Memb. Sci. 2017, 524, 576–584. [CrossRef] 141. Oyetade, O.A.; Skelton, A.A.; Nyamori, V.O.; Jonnalagadda, S.B.; Martincigh, B.S. Experimental and DFT studies on the selective adsorption of Pb2+ and Zn2+ from aqueous solution by nitrogen-functionalized multiwalled carbon nanotubes. Sep. Purif. Technol. 2017, 188, 174–187. [CrossRef] 142. Ali, I. New Generation Adsorbents for Water Treatment. Chem. Rev. 2012, 112, 5073–5091. [CrossRef] 143. Bahgat, M.; Farghali, A.A.; El Rouby, W.M.A.; Khedr, M.H. Synthesis and modification of multi-walled carbon nano-tubes (MWCNTs) for water treatment applications. J. Anal. Appl. Pyrolysis 2011, 92, 307–313. [CrossRef] 144. Zhang, Y.; Wu, B.; Xu, H.; Liu, H.; Wang, M.; He, Y.; Pan, B. Nanomaterials-enabled water and wastewater treatment. Nanoimpact 2016, 3–4, 22–39. [CrossRef] 145. Upadhyayula, V.K.K.; Deng, S.; Mitchell, M.C.; Smith, G.B. Application of carbon nanotube technology for removal of contami- nants in drinking water: A review. Sci. Total Environ. 2009, 408, 1–13. [CrossRef] 146. Rao, G.; Lu, C.; Su, F. Sorption of divalent metal ions from aqueous solution by carbon nanotubes: A review. Sep. Purif. Technol. 2007, 58, 224–231. [CrossRef] 147. Ren, X.; Chen, C.; Nagatsu, M.; Wang, X. Carbon nanotubes as adsorbents in environmental pollution management: A review. Chem. Eng. J. 2011, 170, 395–410. [CrossRef] Sustainability 2021, 13, 5717 48 of 54

148. Liang, J.; Li, L.; Chen, D.; Hajagos, T.; Ren, Z.; Chou, S.-Y.; Hu, W.; Pei, Q. Intrinsically stretchable and transparent thin-film transistors based on printable silver nanowires, carbon nanotubes and an elastomeric dielectric. Nat. Commun. 2015, 6, 7647. [CrossRef] 149. Moghaddam, H.K.; Pakizeh, M. Experimental study on mercury ions removal from aqueous solution by MnO2/CNTs nanocom- posite adsorbent. J. Ind. Eng. Chem. 2015, 21, 221–229. [CrossRef] 150. Tang, W.-W.; Zeng, G.-M.; Gong, J.-L.; Liu, Y.; Wang, X.-Y.; Liu, Y.-Y.; Liu, Z.-F.; Chen, L.; Zhang, X.-R.; Tu, D.-Z. Simultaneous adsorption of atrazine and Cu (II) from wastewater by magnetic multi-walled carbon nanotube. Chem. Eng. J. 2012, 211–212, 470–478. [CrossRef] 151. Lehman, J.H.; Terrones, M.; Mansfield, E.; Hurst, K.E.; Meunier, V. Evaluating the characteristics of multiwall carbon nanotubes. Carbon N. Y. 2011, 49, 2581–2602. [CrossRef] 152. Hassellöv, M.; Readman, J.W.; Ranville, J.F.; Tiede, K. Nanoparticle analysis and characterization methodologies in environmental risk assessment of engineered nanoparticles. Ecotoxicology 2008, 17, 344–361. [CrossRef] 153. Belin, T.; Epron, F. Characterization methods of carbon nanotubes: A review. Mater. Sci. Eng. B 2005, 119, 105–118. [CrossRef] 154. Thostenson, E.T.; Ren, Z.; Chou, T.-W. Advances in the science and technology of carbon nanotubes and their composites: A review. Compos. Sci. Technol. 2001, 61, 1899–1912. [CrossRef] 155. Herrero-Latorre, C.; Álvarez-Méndez, J.; Barciela-García, J.; García-Martín, S.; Peña-Crecente, R.M. Characterization of carbon nanotubes and analytical methods for their determination in environmental and biological samples: A review. Anal. Chim. Acta 2015, 853, 77–94. [CrossRef] 156. Täschner, C.; Pácal, F.; Leonhardt, A.; Spatenka, P.; Bartsch, K.; Graff, A.; Kaltofen, R. Synthesis of aligned carbon nanotubes by DC plasma-enhanced hot filament CVD. Surf. Coat. Technol. 2003, 174–175, 81–87. [CrossRef] 157. Xiao, L.; Ha, J.W.; Wei, L.; Wang, G.; Fang, N. Determining the Full Three-Dimensional Orientation of Single Anisotropic Nanoparticles by Differential Interference Contrast Microscopy. Angew. Chem. 2012, 124, 7854–7858. [CrossRef] 158. Korneva, G.; Ye, H.; Gogotsi, Y.; Halverson, D.; Friedman, G.; Bradley, J.-C.; Kornev, K.G. Carbon Nanotubes Loaded with Magnetic Particles. Nano Lett. 2005, 5, 879–884. [CrossRef] 159. Jia, C.L.; Mi, S.B.; Faley, M.; Poppe, U.; Schubert, J.; Urban, K. Oxygen octahedron reconstruction in the SrTiO3/LaAlO3 heterointerfaces investigated using aberration-corrected ultrahigh-resolution transmission electron microscopy. Phys. Rev. B 2009, 79, 081405. [CrossRef] 160. Gao, C.; Stading, M.; Wellner, N.; Parker, M.L.; Noel, T.R.; Mills, E.N.C.; Belton, P.S. Plasticization of a Protein-Based Film by Glycerol: A Spectroscopic, Mechanical, and Thermal Study. J. Agric. Food Chem. 2006, 54, 4611–4616. [CrossRef][PubMed] 161. Sun, Y.-P.; Fu, K.; Lin, Y.; Huang, W. Functionalized Carbon Nanotubes: Properties and Applications. Acc. Chem. Res. 2002, 35, 1096–1104. [CrossRef][PubMed] 162. Jin, S.; Fallgren, P.H.; Morris, J.M.; Chen, Q. Removal of bacteria and viruses from waters using layered double hydroxide nanocomposites. Sci. Technol. Adv. Mater. 2007, 8, 67–70. [CrossRef] 163. Huang, J.; Cao, Y.; Qin, B.; Zhong, G.; Zhang, J.; Yu, H.; Wang, H.; Peng, F. Highly efficient and acid-corrosion resistant nitrogen doped magnetic carbon nanotubes for the hexavalent chromium removal with subsequent reutilization. Chem. Eng. J. 2019, 361, 547–558. [CrossRef] 164. Correa-Duarte, M.A.; Grzelczak, M.; Salgueiriño-Maceira, V.; Giersig, M.; Liz-Marzán, L.M.; Farle, M.; Sierazdki, K.; Diaz, R. Alignment of Carbon Nanotubes under Low Magnetic Fields through Attachment of Magnetic Nanoparticles. J. Phys. Chem. B 2005, 109, 19060–19063. [CrossRef] 165. Rinzler, A.G.; Liu, J.; Dai, H.; Nikolaev, P.; Huffman, C.B.; Rodríguez-Macías, F.J.; Boul, P.J.; Lu, A.H.; Heymann, D.; Colbert, D.T.; et al. Large-scale purification of single-wall carbon nanotubes: Process, product, and characterization. Appl. Phys. A Mater. Sci. Process. 1998, 67, 29–37. [CrossRef] 166. Gommes, C.; Blacher, S.; Masenelli-Varlot, K.; Bossuot, C.; McRae, E.; Fonseca, A.; Nagy, J.-B.; Pirard, J.-P. Image analysis characterization of multi-walled carbon nanotubes. Carbon N. Y. 2003, 41, 2561–2572. [CrossRef] 167. Jimeno, A.; Goyanes, S.; Eceiza, A.; Kortaberria, G.; Mondragon, I.; Corcuera, M.A. Effects of Amine Molecular Structure on Carbon Nanotubes Functionalization. J. Nanosci. Nanotechnol. 2009, 9, 6222–6227. [CrossRef] 168. Chai, S.-P.; Zein, S.H.S.; Mohamed, A.R. The effect of reduction temperature on Co-Mo/Al2O3 catalysts for carbon nanotubes formation. Appl. Catal. A Gen. 2007, 326, 173–179. [CrossRef] 169. Kiang, C.-H.; Endo, M.; Ajayan, P.M.; Dresselhaus, G.; Dresselhaus, M.S. Size Effects in Carbon Nanotubes. Phys. Rev. Lett. 1998, 81, 1869–1872. [CrossRef] 170. Sarkar, C.; Chowdhuri, A.R.; Kumar, A.; Laha, D.; Garai, S.; Chakraborty, J.; Sahu, S.K. One pot synthesis of carbon dots decorated carboxymethyl cellulose- hydroxyapatite nanocomposite for drug delivery, tissue engineering and Fe3+ ion sensing. Carbohydr. Polym. 2018, 181, 710–718. [CrossRef] 171. Wepasnick, K.A.; Smith, B.A.; Bitter, J.L.; Howard Fairbrother, D. Chemical and structural characterization of carbon nanotube surfaces. Anal. Bioanal. Chem. 2010, 396, 1003–1014. [CrossRef] 172. Datsyuk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Chemical oxidation of multiwalled carbon nanotubes. Carbon N. Y. 2008, 46, 833–840. [CrossRef] 173. Liu, M.; Cowley, J.M. Structures of carbon nanotubes studied by HRTEM and nanodiffraction. Ultramicroscopy 1994, 53, 333–342. [CrossRef] Sustainability 2021, 13, 5717 49 of 54

174. Cao, A.; Zhang, X.; Xu, C.; Wei, B.; Wu, D. Tandem structure of aligned carbon nanotubes on Au and its solar thermal absorption. Sol. Energy Mater. Sol. Cells 2002, 70, 481–486. [CrossRef] 175. Droppa, R.; Hammer, P.; Carvalho, A.C.; dos Santos, M.; Alvarez, F. Incorporation of nitrogen in carbon nanotubes. J. Non-Cryst. Solids 2002, 299–302, 874–879. [CrossRef] 176. Chong, C.T.; Tan, W.H.; Lee, S.L.; Chong, W.W.F.; Lam, S.S.; Valera-Medina, A. Morphology and growth of carbon nanotubes catalytically synthesised by premixed hydrocarbon-rich flames. Mater. Chem. Phys. 2017, 197, 246–255. [CrossRef] 177. Hammer, P.; Victoria, N.M.; Alvarez, F. Effects of increasing nitrogen concentration on the structure of carbon nitride films deposited by ion beam assisted deposition. J. Vac. Sci. Technol. A Vac. Surfaces Film. 2000, 18, 2277. [CrossRef] 178. Pham-Huu, C.; Keller, N.; Roddatis, V.V.; Mestl, G.; Schlögl, R.; Ledoux, M.J. Large scale synthesis of carbon nanofibers by catalytic decomposition of ethane on nickel nanoclusters decorating carbon nanotubes. Phys. Chem. Chem. Phys. 2002, 4, 514–521. [CrossRef] 179. Lee, Y.S.; Cho, T.H.; Lee, B.K.; Rho, J.S.; An, K.H.; Lee, Y.H. Surface properties of fluorinated single-walled carbon nanotubes. J. Fluor. Chem. 2003, 120, 99–104. [CrossRef] 180. Cao, A.; Xu, C.; Liang, J.; Wu, D.; Wei, B. X-ray diffraction characterization on the alignment degree of carbon nanotubes. Chem. Phys. Lett. 2001, 344, 13–17. [CrossRef] 181. Rols, S.; Almairac, R.; Henrard, L.; Anglaret, E.; Sauvajol, J.-L. Diffraction by finite-size crystalline bundles of single wall nanotubes. Eur. Phys. J. B 1999, 10, 263–270. [CrossRef] 182. Kuzmany, H.; Plank, W.; Hulman, M.; Kramberger, C.; Grüneis, A.; Pichler, T.; Peterlik, H.; Kataura, H.; Achiba, Y. Determination of SWCNT diameters from the Raman response of the radial breathing mode. Eur. Phys. J. B 2001, 22, 307–320. [CrossRef] 183. Santos, T.C.; dos Gates, R.S.; de Tinôco, I.F.F.; Zolnier, S.; da Baêta, F.C. Behavior of Japanese quail in different air velocities and air temperatures. Pesqui. Agropecuária Bras. 2017, 52, 344–354. [CrossRef] 184. Cividanes, L.S.; Brunelli, D.D.; Antunes, E.F.; Corat, E.J.; Sakane, K.K.; Thim, G.P. Cure study of epoxy resin reinforced with multiwalled carbon nanotubes by Raman and luminescence spectroscopy. J. Appl. Polym. Sci. 2013, 127, 544–553. [CrossRef] 185. Puangjan, A.; Chaiyasith, S.; Taweeporngitgul, W.; Keawtep, J. Application of functionalized multi-walled carbon nanotubes supporting cuprous oxide and silver oxide composite catalyst on copper substrate for simultaneous detection of vitamin B2, vitamin B6 and ascorbic acid. Mater. Sci. Eng. C 2017, 76, 383–397. [CrossRef] 186. Ferreira, F.V.; Francisco, W.; Menezes, B.R.C.; Brito, F.S.; Coutinho, A.S.; Cividanes, L.S.; Coutinho, A.R.; Thim, G.P. Correlation of surface treatment, dispersion and mechanical properties of HDPE/CNT nanocomposites. Appl. Surf. Sci. 2016, 389, 921–929. [CrossRef] 187. Silambarasan, D.; Surya, V.J.; Iyakutti, K.; Asokan, K.; Vasu, V.; Kawazoe, Y. Gamma (γ)-ray irradiated multi-walled carbon nanotubes (MWCNTs) for hydrogen storage. Appl. Surf. Sci. 2017, 418, 49–55. [CrossRef] 188. Arepalli, S.; Nikolaev, P.; Gorelik, O.; Hadjiev, V.G.; Holmes, W.; Files, B.; Yowell, L. Protocol for the characterization of single-wall carbon nanotube material quality. Carbon N. Y. 2004, 42, 1783–1791. [CrossRef] 189. Alvarez, W.E.; Kitiyanan, B.; Borgna, A.; Resasco, D.E. Synergism of Co and Mo in the catalytic production of single-wall carbon nanotubes by decomposition of CO. Carbon N. Y. 2001, 39, 547–558. [CrossRef] 190. Bahr, J.L.; Yang, J.; Kosynkin, D.V.; Bronikowski, M.J.; Smalley, R.E.; Tour, J.M. Functionalization of Carbon Nanotubes by Electrochemical Reduction of Aryl Diazonium Salts: A Bucky Paper Electrode. J. Am. Chem. Soc. 2001, 123, 6536–6542. [CrossRef] 191. Liu, W.-W.; Aziz, A.; Chai, S.-P.; Mohamed, A.R.; Tye, C.-T. Preparation of iron oxide nanoparticles supported on magnesium oxide for producing high-quality single-walled carbon nanotubes. New Carbon Mater. 2011, 26, 255–261. [CrossRef] 192. Anoshkin, I.V.; Nefedova, I.I.; Lioubtchenko, D.V.; Nefedov, I.S.; Räisänen, A.V. Single walled carbon nanotube quantification method employing the Raman signal intensity. Carbon N. Y. 2017, 116, 547–552. [CrossRef] 193. Xue, Y.; Zheng, S.; Sun, Z.; Zhang, Y.; Jin, W. Alkaline electrochemical advanced oxidation process for chromium oxidation at graphitized multi-walled carbon nanotubes. Chemosphere 2017, 183, 156–163. [CrossRef] 194. Faraji, S.; Yildiz, O.; Rost, C.; Stano, K.; Farahbakhsh, N.; Zhu, Y.; Bradford, P.D. Radial growth of multi-walled carbon nanotubes in aligned sheets through cyclic carbon deposition and graphitization. Carbon N. Y. 2017, 111, 411–418. [CrossRef] 195. Pillay, K.; Cukrowska, E.M.; Coville, N.J. Multi-walled carbon nanotubes as adsorbents for the removal of parts per billion levels of hexavalent chromium from aqueous solution. J. Hazard. Mater. 2009, 166, 1067–1075. [CrossRef] 196. Yu, F.; Ma, J.; Wang, J.; Zhang, M.; Zheng, J. Magnetic iron oxide nanoparticles functionalized multi-walled carbon nanotubes for toluene, ethylbenzene and xylene removal from aqueous solution. Chemosphere 2016, 146, 162–172. [CrossRef] 197. Usman Farid, M.; Luan, H.-Y.; Wang, Y.; Huang, H.; An, A.K.; Jalil Khan, R. Increased adsorption of aqueous zinc species by Ar/O 2 plasma-treated carbon nanotubes immobilized in hollow-fiber ultrafiltration membrane. Chem. Eng. J. 2017, 325, 239–248. [CrossRef] 198. Agnihotri, S.; Mota, J.P.B.; Rostam-Abadi, M.; Rood, M.J. Theoretical and Experimental Investigation of Morphology and Temperature Effects on Adsorption of Organic Vapors in Single-Walled Carbon Nanotubes. J. Phys. Chem. B 2006, 110, 7640–7647. [CrossRef] 199. Cho, H.-H.; Wepasnick, K.; Smith, B.A.; Bangash, F.K.; Fairbrother, D.H.; Ball, W.P. Sorption of Aqueous Zn[II] and Cd[II] by Multiwall Carbon Nanotubes: The Relative Roles of Oxygen-Containing Functional Groups and Graphenic Carbon. Langmuir 2010, 26, 967–981. [CrossRef] Sustainability 2021, 13, 5717 50 of 54

200. Yu, X.-Y.; Luo, T.; Zhang, Y.-X.; Jia, Y.; Zhu, B.-J.; Fu, X.-C.; Liu, J.-H.; Huang, X.-J. Adsorption of Lead(II) on O2-Plasma-Oxidized Multiwalled Carbon Nanotubes: Thermodynamics, Kinetics, and Desorption. ACS Appl. Mater. Interfaces 2011, 3, 2585–2593. [CrossRef] 201. Addo Ntim, S.; Mitra, S. Removal of Trace Arsenic To Meet Drinking Water Standards Using Iron Oxide Coated Multiwall Carbon Nanotubes. J. Chem. Eng. Data 2011, 56, 2077–2083. [CrossRef][PubMed] 202. Chen, C.; Hu, J.; Shao, D.; Li, J.; Wang, X. Adsorption behavior of multiwall carbon nanotube/iron oxide magnetic composites for Ni(II) and Sr(II). J. Hazard. Mater. 2009, 164, 923–928. [CrossRef][PubMed] 203. Daneshvar Tarigh, G.; Shemirani, F. Magnetic multi-wall carbon nanotube nanocomposite as an adsorbent for preconcentration and determination of lead (II) and manganese (II) in various matrices. Talanta 2013, 115, 744–750. [CrossRef] 204. Zhao, X.; Jia, Q.; Song, N.; Zhou, W.; Li, Y. Adsorption of Pb(II) from an Aqueous Solution by Titanium Dioxide/Carbon Nanotube Nanocomposites: Kinetics, Thermodynamics, and Isotherms. J. Chem. Eng. Data 2010, 55, 4428–4433. [CrossRef] 205. Gupta, A.; Vidyarthi, S.R.; Sankararamakrishnan, N. Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled carbon nanotubes. J. Hazard. Mater. 2014, 274, 132–144. [CrossRef] 206. Gupta, V.K.; Moradi, O.; Tyagi, I.; Agarwal, S.; Sadegh, H.; Shahryari-Ghoshekandi, R.; Makhlouf, A.S.H.; Goodarzi, M.; Garshasbi, A. Study on the removal of heavy metal ions from industry waste by carbon nanotubes: Effect of the surface modification: A review. Crit. Rev. Environ. Sci. Technol. 2016, 46, 93–118. [CrossRef] 207. Hu, J.; Zhao, D.; Wang, X. Removal of Pb(II) and Cu(II) from aqueous solution using multiwalled carbon nanotubes/iron oxide magnetic composites. Water Sci. Technol. 2011, 63, 917–923. [CrossRef] 208. Yang, S.; Guo, Z.; Sheng, G.; Wang, X. Investigation of the sequestration mechanisms of Cd(II) and 1-naphthol on discharged multi-walled carbon nanotubes in aqueous environment. Sci. Total Environ. 2012, 420, 214–221. [CrossRef] 209. Vukovi´c,G.D.; Marinkovi´c,A.D.; Coli´c,M.;ˇ Risti´c,M.Ð.; Aleksi´c,R.; Peri´c-Gruji´c,A.A.; Uskokovi´c,P.S. Removal of cadmium from aqueous solutions by oxidized and ethylenediamine-functionalized multi-walled carbon nanotubes. Chem. Eng. J. 2010, 157, 238–248. [CrossRef] 210. Stafiej, A.; Pyrzynska, K. Solid phase extraction of metal ions using carbon nanotubes. Microchem. J. 2008, 89, 29–33. [CrossRef] 211. Lu, C.; Chiu, H. Adsorption of zinc(II) from water with purified carbon nanotubes. Chem. Eng. Sci. 2006, 61, 1138–1145. [CrossRef] 212. Weng, C.-H.; Huang, C.P. Adsorption characteristics of Zn(II) from dilute aqueous solution by fly ash. Colloids Surf. A Physicochem. Eng. Asp. 2004, 247, 137–143. [CrossRef] 213. Boehm, H. Surface oxides on carbon and their analysis: A critical assessment. Carbon N. Y. 2002, 40, 145–149. [CrossRef] 214. Li, J.; Chen, S.; Sheng, G.; Hu, J.; Tan, X.; Wang, X. Effect of surfactants on Pb(II) adsorption from aqueous solutions using oxidized multiwall carbon nanotubes. Chem. Eng. J. 2011, 166, 551–558. [CrossRef] 215. Atieh, M.A.; Bakather, O.Y.; Tawabini, B.S.; Bukhari, A.A.; Khaled, M.; Alharthi, M.; Fettouhi, M.; Abuilaiwi, F.A. Removal of Chromium (III) from Water by Using Modified and Nonmodified Carbon Nanotubes. J. Nanomater. 2010, 2010, 1–9. [CrossRef] 216. Lu, C.; Liu, C.; Rao, G.P. Comparisons of sorbent cost for the removal of Ni2+ from aqueous solution by carbon nanotubes and granular activated carbon. J. Hazard. Mater. 2008, 151, 239–246. [CrossRef] 217. Ali, I. Microwave assisted economic synthesis of multi walled carbon nanotubes for arsenic species removal in water: Batch and column operations. J. Mol. Liq. 2018, 271, 677–685. [CrossRef] 218. Alijani, H.; Shariatinia, Z. Effective aqueous arsenic removal using zero valent iron doped MWCNT synthesized by in situ CVD method using natural α-Fe2O3 as a precursor. Chemosphere 2017, 171, 502–511. [CrossRef] 219. Sankararamakrishnan, N.; Chauhan, D.; Dwivedi, J. Synthesis of functionalized carbon nanotubes by floating catalytic chemical vapor deposition method and their sorption behavior toward arsenic. Chem. Eng. J. 2016, 284, 599–608. [CrossRef] 220. Ma, M.-D.; Wu, H.; Deng, Z.-Y.; Zhao, X. Arsenic removal from water by nanometer iron oxide coated single-wall carbon nanotubes. J. Mol. Liq. 2018, 259, 369–375. [CrossRef] 221. Peng, H.; Zhang, N.; He, M.; Chen, B.; Hu, B. Simultaneous speciation analysis of inorganic arsenic, chromium and selenium in environmental waters by 3-(2-aminoethylamino) propyltrimethoxysilane modified multi-wall carbon nanotubes packed microcolumn solid phase extraction and ICP-MS. Talanta 2015, 131, 266–272. [CrossRef] 222. Veliˇckovi´c,Z.; Vukovi´c,G.D.; Marinkovi´c,A.D.; Moldovan, M.-S.; Peri´c-Gruji´c,A.A.; Uskokovi´c,P.S.; Risti´c,M.Ð. Adsorption of arsenate on iron(III) oxide coated ethylenediamine functionalized multiwall carbon nanotubes. Chem. Eng. J. 2012, 181–182, 174–181. [CrossRef] 223. Addo Ntim, S.; Mitra, S. Adsorption of arsenic on multiwall carbon nanotube–zirconia nanohybrid for potential drinking water purification. J. Colloid Interface Sci. 2012, 375, 154–159. [CrossRef] 224. De Marques Neto, J.O.; Bellato, C.R.; de Silva, D.C. Iron oxide/carbon nanotubes/chitosan magnetic composite film for chromium species removal. Chemosphere 2019, 218, 391–401. [CrossRef] 225. Shin, K.-Y.; Hong, J.-Y.; Jang, J. Heavy metal ion adsorption behavior in nitrogen-doped magnetic carbon nanoparticles: Isotherms and kinetic study. J. Hazard. Mater. 2011, 190, 36–44. [CrossRef] 226. Huang, Y.; Lee, X.; Macazo, F.C.; Grattieri, M.; Cai, R.; Minteer, S.D. Fast and efficient removal of chromium (VI) anionic species by a reusable chitosan-modified multi-walled carbon nanotube composite. Chem. Eng. J. 2018, 339, 259–267. [CrossRef] 227. Lu, W.; Li, J.; Sheng, Y.; Zhang, X.; You, J.; Chen, L. One-pot synthesis of magnetic iron oxide nanoparticle-multiwalled carbon nanotube composites for enhanced removal of Cr(VI) from aqueous solution. J. Colloid Interface Sci. 2017, 505, 1134–1146. [CrossRef] Sustainability 2021, 13, 5717 51 of 54

228. Salam, M.A. Preparation and characterization of chitin/magnetite/multiwalled carbon nanotubes magnetic nanocomposite for toxic hexavalent chromium removal from solution. J. Mol. Liq. 2017, 233, 197–202. [CrossRef] 229. Huang, Z.; Wang, X.; Yang, D. Adsorption of Cr(VI) in wastewater using magnetic multi-wall carbon nanotubes. Water Sci. Eng. 2015, 8, 226–232. [CrossRef] 230. Di, Z.-C.; Ding, J.; Peng, X.-J.; Li, Y.-H.; Luan, Z.-K.; Liang, J. Chromium adsorption by aligned carbon nanotubes supported ceria nanoparticles. Chemosphere 2006, 62, 861–865. [CrossRef][PubMed] 231. Zhang, C.; Sui, J.; Li, J.; Tang, Y.; Cai, W. Efficient removal of heavy metal ions by thiol-functionalized superparamagnetic carbon nanotubes. Chem. Eng. J. 2012, 210, 45–52. [CrossRef] 232. Wang, G.; Gao, Z.; Tang, S.; Chen, C.; Duan, F.; Zhao, S.; Lin, S.; Feng, Y.; Zhou, L.; Qin, Y. Microwave Absorption Properties of Carbon Nanocoils Coated with Highly Controlled Magnetic Materials by Atomic Layer Deposition. ACS Nano 2012, 6, 11009–11017. [CrossRef][PubMed] 233. Ji, L.; Zhou, L.; Bai, X.; Shao, Y.; Zhao, G.; Qu, Y.; Wang, C.; Li, Y. Facile synthesis of multiwall carbon nanotubes/iron oxides for removal of tetrabromobisphenol A and Pb(ii). J. Mater. Chem. 2012, 22, 15853. [CrossRef] 234. Yang, L.; Jiang, S.; Zhao, Y.; Zhu, L.; Chen, S.; Wang, X.; Wu, Q.; Ma, J.; Ma, Y.; Hu, Z. Boron-Doped Carbon Nanotubes as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2011, 50, 7132–7135. [CrossRef][PubMed] 235. Tofighy, M.A.; Mohammadi, T. Adsorption of divalent heavy metal ions from water using carbon nanotube sheets. J. Hazard. Mater. 2011, 185, 140–147. [CrossRef] 236. Ren, X.; Shao, D.; Zhao, G.; Sheng, G.; Hu, J.; Yang, S.; Wang, X. Plasma Induced Multiwalled Carbon Nanotube Grafted with 2-Vinylpyridine for Preconcentration of Pb(II) from Aqueous Solutions. Plasma Process. Polym. 2011, 8, 589–598. [CrossRef] 237. Xu, D.; Wang, Z. Role of multi-wall carbon nanotube network in composites to crystallization of isotactic polypropylene matrix. Polymer 2008, 49, 330–338. [CrossRef] 238. Wang, S.; Gong, W.; Liu, X.; Yao, Y.; Gao, B.; Yue, Q. Removal of lead(II) from aqueous solution by adsorption onto manganese oxide-coated carbon nanotubes. Sep. Purif. Technol. 2007, 58, 17–23. [CrossRef] 239. Wang, H.J.; Zhou, A.L.; Peng, F.; Yu, H.; Chen, L.F. Adsorption characteristic of acidified carbon nanotubes for heavy metal Pb(II) in aqueous solution. Mater. Sci. Eng. A 2007, 466, 201–206. [CrossRef] 240. Liang, J.; Liu, J.; Yuan, X.; Dong, H.; Zeng, G.; Wu, H.; Wang, H.; Liu, J.; Hua, S.; Zhang, S.; et al. Facile synthesis of alumina- decorated multi-walled carbon nanotubes for simultaneous adsorption of cadmium ion and trichloroethylene. Chem. Eng. J. 2015, 273, 101–110. [CrossRef] 241. Al-Khaldi, F.A.; Abusharkh, B.; Khaled, M.; Atieh, M.A.; Nasser, M.S.; Saleh, T.A.; Agarwal, S.; Tyagi, I.; Gupta, V.K. Adsorptive removal of cadmium(II) ions from liquid phase using acid modified carbon-based adsorbents. J. Mol. Liq. 2015, 204, 255–263. [CrossRef] 242. Salam, M.A.; Makki, M.S.I.; Abdelaal, M.Y.A. Preparation and characterization of multi-walled carbon nanotubes/chitosan nanocomposite and its application for the removal of heavy metals from aqueous solution. J. Alloys Compd. 2011, 509, 2582–2587. [CrossRef] 243. Bandaru, N.M.; Reta, N.; Dalal, H.; Ellis, A.V.; Shapter, J.; Voelcker, N.H. Enhanced adsorption of mercury ions on thiol derivatized single wall carbon nanotubes. J. Hazard. Mater. 2013, 261, 534–541. [CrossRef] 244. Hadavifar, M.; Bahramifar, N.; Younesi, H.; Li, Q. Adsorption of mercury ions from synthetic and real wastewater aqueous solution by functionalized multi-walled carbon nanotube with both amino and thiolated groups. Chem. Eng. J. 2014, 237, 217–228. [CrossRef] 245. Pillay, K.; Cukrowska, E.M.; Coville, N.J. Improved uptake of mercury by sulphur-containing carbon nanotubes. Microchem. J. 2013, 108, 124–130. [CrossRef] 246. El-Sheikh, A.H.; Al-Degs, Y.S.; Al-As’ad, R.M.; Sweileh, J.A. Effect of oxidation and geometrical dimensions of carbon nanotubes on Hg(II) sorption and preconcentration from real waters. Desalination 2011, 270, 214–220. [CrossRef] 247. Mubarak, N.M.; Alicia, R.F.; Abdullah, E.C.; Sahu, J.N.; Haslija, A.B.A.; Tan, J. Statistical optimization and kinetic studies on removal of Zn2+ using functionalized carbon nanotubes and magnetic biochar. J. Environ. Chem. Eng. 2013, 1, 486–495. [CrossRef] 248. Ge, Y.; Li, Z.; Xiao, D.; Xiong, P.; Ye, N. Sulfonated multi-walled carbon nanotubes for the removal of copper (II) from aqueous solutions. J. Ind. Eng. Chem. 2014, 20, 1765–1771. [CrossRef] 249. Salehi, E.; Madaeni, S.S.; Rajabi, L.; Vatanpour, V.; Derakhshan, A.A.; Zinadini, S.; Ghorabi, S.; Ahmadi Monfared, H. Novel chitosan/poly(vinyl) alcohol thin adsorptive membranes modified with amino functionalized multi-walled carbon nanotubes for Cu(II) removal from water: Preparation, characterization, adsorption kinetics and thermodynamics. Sep. Purif. Technol. 2012, 89, 309–319. [CrossRef] 250. Shao, D.; Hu, J.; Chen, C.; Sheng, G.; Ren, X.; Wang, X. Polyaniline Multiwalled Carbon Nanotube Magnetic Composite Prepared by Plasma-Induced Graft Technique and Its Application for Removal of Aniline and Phenol. J. Phys. Chem. C 2010, 114, 21524–21530. [CrossRef] 251. Chen, H.; Li, J.; Shao, D.; Ren, X.; Wang, X. Poly(acrylic acid) grafted multiwall carbon nanotubes by plasma techniques for Co(II) removal from aqueous solution. Chem. Eng. J. 2012, 210, 475–481. [CrossRef] 252. Wang, Q.; Li, J.; Chen, C.; Ren, X.; Hu, J.; Wang, X. Removal of cobalt from aqueous solution by magnetic multiwalled carbon nanotube/iron oxide composites. Chem. Eng. J. 2011, 174, 126–133. [CrossRef] Sustainability 2021, 13, 5717 52 of 54

253. Mobasherpour, I.; Salahi, E.; Ebrahimi, M. Removal of divalent nickel cations from aqueous solution by multi-walled carbon nano tubes: Equilibrium and kinetic processes. Res. Chem. Intermed. 2012, 38, 2205–2222. [CrossRef] 254. Kandah, M.I.; Meunier, J.-L. Removal of nickel ions from water by multi-walled carbon nanotubes. J. Hazard. Mater. 2007, 146, 283–288. [CrossRef] 255. Chen, C.; Wang, X. Adsorption of Ni(II) from Aqueous Solution Using Oxidized Multiwall Carbon Nanotubes. Ind. Eng. Chem. Res. 2006, 45, 9144–9149. [CrossRef] 256. Abdel-Ghani, N.T.; El-Chaghaby, G.A.; Helal, F.S. Individual and competitive adsorption of phenol and nickel onto multiwalled carbon nanotubes. J. Adv. Res. 2015, 6, 405–415. [CrossRef] 257. Deb, A.K.S.; Ilaiyaraja, P.; Ponraju, D.; Venkatraman, B. Diglycolamide functionalized multi-walled carbon nanotubes for removal of uranium from aqueous solution by adsorption. J. Radioanal. Nucl. Chem. 2012, 291, 877–883. [CrossRef] 258. Chen, C.; Hu, J.; Xu, D.; Tan, X.; Meng, Y.; Wang, X. Surface complexation modeling of Sr(II) and Eu(III) adsorption onto oxidized multiwall carbon nanotubes. J. Colloid Interface Sci. 2008, 323, 33–41. [CrossRef] 259. Tousova, Z.; Oswald, P.; Slobodnik, J.; Blaha, L.; Muz, M.; Hu, M.; Brack, W.; Krauss, M.; Di Paolo, C.; Tarcai, Z.; et al. European demonstration program on the effect-based and chemical identification and monitoring of organic pollutants in European surface waters. Sci. Total Environ. 2017, 601–602, 1849–1868. [CrossRef] 260. Sillanpää, M. Natural Organic Matter in Water, 1st ed.; Elsevier: Amsterdam, The Netherlands, 2015; ISBN 9780128015032. 261. Zare, K.; Gupta, V.K.; Moradi, O.; Makhlouf, A.S.H.; Sillanpää, M.; Nadagouda, M.N.; Sadegh, H.; Shahryari-ghoshekandi, R.; Pal, A.; Wang, Z.; et al. A comparative study on the basis of adsorption capacity between CNTs and activated carbon as adsorbents for removal of noxious synthetic dyes: A review. J. Nanostruct. Chem. 2015, 5, 227–236. [CrossRef] 262. Ghaedi, M.; Khajehsharifi, H.; Yadkuri, A.H.; Roosta, M.; Asghari, A. Oxidized multiwalled carbon nanotubes as efficient adsorbent for bromothymol blue. Toxicol. Environ. Chem. 2012, 94, 873–883. [CrossRef] 263. Mahmoodian, H.; Moradi, O.; Shariatzadeha, B.; Salehf, T.A.; Tyagi, I.; Maity, A.; Asif, M.; Gupta, V.K. Enhanced removal of methyl orange from aqueous solutions by poly HEMA–chitosan-MWCNT nano-composite. J. Mol. Liq. 2015, 202, 189–198. [CrossRef] 264. Duman, O.; Tunç, S.; Polat, T.G.; Bozo˘glan,B.K. Synthesis of magnetic oxidized multiwalled carbon nanotube-κ-carrageenan-Fe 3 O 4 nanocomposite adsorbent and its application in cationic Methylene Blue dye adsorption. Carbohydr. Polym. 2016, 147, 79–88. [CrossRef] 265. Sadegh, H.; Zare, K.; Maazinejad, B.; Shahryari-ghoshekandi, R.; Tyagi, I.; Agarwal, S.; Gupta, V.K. Synthesis of MWCNT-COOH- Cysteamine composite and its application for dye removal. J. Mol. Liq. 2016, 215, 221–228. [CrossRef] 266. Wang, Y.; Huang, H.; Wei, X. Influence of wastewater precoagulation on adsorptive filtration of pharmaceutical and personal care products by carbon nanotube membranes. Chem. Eng. J. 2018, 333, 66–75. [CrossRef] 267. Wang, S.; Liang, S.; Liang, P.; Zhang, X.; Sun, J.; Wu, S.; Huang, X. In-situ combined dual-layer CNT/PVDF membrane for electrically-enhanced fouling resistance. J. Memb. Sci. 2015, 491, 37–44. [CrossRef] 268. Wang, Y.; Zhu, J.; Huang, H.; Cho, H.-H. Carbon nanotube composite membranes for microfiltration of pharmaceuticals and personal care products: Capabilities and potential mechanisms. J. Memb. Sci. 2015, 479, 165–174. [CrossRef] 269. Jahangiri-Rad, M.; Nadafi, K.; Mesdaghinia, A.; Nabizadeh, R.; Younesian, M.; Rafiee, M. Sequential study on reactive blue 29 dye removal from aqueous solution by peroxy acid and single wall carbon nanotubes: Experiment and theory. Iran. J. Environ. Health Sci. Eng. 2013, 10, 5. [CrossRef][PubMed] 270. Engel, M.; Chefetz, B. Adsorption and desorption of dissolved organic matter by carbon nanotubes: Effects of solution chemistry. Environ. Pollut. 2016, 213, 90–98. [CrossRef] 271. Ajmani, G.S.; Goodwin, D.; Marsh, K.; Fairbrother, D.H.; Schwab, K.J.; Jacangelo, J.G.; Huang, H. Modification of low pressure membranes with carbon nanotube layers for fouling control. Water Res. 2012, 46, 5645–5654. [CrossRef][PubMed] 272. Yang, X.; Lee, J.; Yuan, L.; Chae, S.-R.; Peterson, V.K.; Minett, A.I.; Yin, Y.; Harris, A.T. Removal of natural organic matter in water using functionalised carbon nanotube buckypaper. Carbon N. Y. 2013, 59, 160–166. [CrossRef] 273. Deng, J.; Shao, Y.; Gao, N.; Deng, Y.; Tan, C.; Zhou, S.; Hu, X. Multiwalled carbon nanotubes as adsorbents for removal of herbicide diuron from aqueous solution. Chem. Eng. J. 2012, 193–194, 339–347. [CrossRef] 274. Hamdi, H.; De La Torre-Roche, R.; Hawthorne, J.; White, J.C. Impact of non-functionalized and amino-functionalized multiwall carbon nanotubes on pesticide uptake by lettuce (Lactuca sativa L.). Nanotoxicology 2015, 9, 172–180. [CrossRef][PubMed] 275. Rocha, J.-D.R.; Rogers, R.E.; Dichiara, A.B.; Capasse, R.C. Emerging investigators series: Highly effective adsorption of organic aromatic molecules from aqueous environments by electronically sorted single-walled carbon nanotubes. Environ. Sci. Water Res. Technol. 2017, 3, 203–212. [CrossRef] 276. Dichiara, A.B.; Harlander, S.F.; Rogers, R.E. Fixed bed adsorption of diquat dibromide from aqueous solution using carbon nanotubes. RSC Adv. 2015, 5, 61508–61512. [CrossRef] 277. Srivastava, A.; Srivastava, O.N.; Talapatra, S.; Vajtai, R.; Ajayan, P.M. Carbon nanotube filters. Nat. Mater. 2004, 3, 610–614. [CrossRef] 278. Tahaikt, M.; El Habbani, R.; Ait Haddou, A.; Achary, I.; Amor, Z.; Taky, M.; Alami, A.; Boughriba, A.; Hafsi, M.; Elmidaoui, A. Fluoride removal from groundwater by nanofiltration. Desalination 2007, 212, 46–53. [CrossRef] 279. Fornasiero, F.; Park, H.G.; Holt, J.K.; Stadermann, M.; Grigoropoulos, C.P.; Noy, A.; Bakajin, O. Ion exclusion by sub-2-nm carbon nanotube pores. Proc. Natl. Acad. Sci. USA 2008, 105, 17250–17255. [CrossRef] Sustainability 2021, 13, 5717 53 of 54

280. Noy, A.; Park, H.G.; Fornasiero, F.; Holt, J.K.; Grigoropoulos, C.P.; Bakajin, O. Nanofluidics in carbon nanotubes. Nano Today 2007, 2, 22–29. [CrossRef] 281. Hummer, G.; Rasaiah, J.C.; Noworyta, J.P. Water conduction through the hydrophobic channel of a carbon nanotube. Nature 2001, 414, 188–190. [CrossRef][PubMed] 282. Moradi, O. Adsorption Behavior of Basic Red 46 by Single-Walled Carbon Nanotubes Surfaces. Fuller. Nanotub. Carbon Nanostructures 2013, 21, 286–301. [CrossRef] 283. Yang, S.; Wang, L.; Zhang, X.; Yang, W.; Song, G. Enhanced adsorption of Congo red dye by functionalized carbon nanotube/mixed metal oxides nanocomposites derived from layered double hydroxide precursor. Chem. Eng. J. 2015, 275, 315–321. [CrossRef] 284. Konicki, W.; Pełech, I.; Mijowska, E.; Jasi´nska,I. Adsorption of anionic dye Direct Red 23 onto magnetic multi-walled carbon nanotubes-Fe3C nanocomposite: Kinetics, equilibrium and thermodynamics. Chem. Eng. J. 2012, 210, 87–95. [CrossRef] 285. Setareh Derakhshan, M.; Moradi, O. The study of thermodynamics and kinetics methyl orange and malachite green by SWCNTs, SWCNT-COOH and SWCNT-NH2 as adsorbents from aqueous solution. J. Ind. Eng. Chem. 2014, 20, 3186–3194. [CrossRef] 286. Zhao, D.; Zhang, W.; Chen, C.; Wang, X. Adsorption of Methyl Orange Dye Onto Multiwalled Carbon Nanotubes. Procedia Environ. Sci. 2013, 18, 890–895. [CrossRef] 287. Ahmad, A.; Razali, M.H.; Mamat, M.; Mehamod, F.S.B.; Anuar Mat Amin, K. Adsorption of methyl orange by synthesized and functionalized-CNTs with 3-aminopropyltriethoxysilane loaded TiO2 nanocomposites. Chemosphere 2017, 168, 474–482. [CrossRef] 288. Robati, D.; Mirza, B.; Ghazisaeidi, R.; Rajabi, M.; Moradi, O.; Tyagi, I.; Agarwal, S.; Gupta, V.K. Adsorption behavior of methylene blue dye on nanocomposite multi-walled carbon nanotube functionalized thiol (MWCNT-SH) as new adsorbent. J. Mol. Liq. 2016, 216, 830–835. [CrossRef] 289. Shih, M.-W.; Chin, C.-J.M.; Yu, Y.-L. The role of oxygen-containing groups on the adsorption of bisphenol-A on multi-walled carbon nanotube modified by HNO3 and KOH. Process Saf. Environ. Prot. 2017, 112, 308–314. [CrossRef] 290. Kuo, C.-Y. Comparison with as-grown and microwave modified carbon nanotubes to removal aqueous bisphenol A. Desalination 2009, 249, 976–982. [CrossRef] 291. Liao, Q.; Sun, J.; Gao, L. Adsorption of chlorophenols by multi-walled carbon nanotubes treated with HNO3 and NH3. Carbon N. Y. 2008, 46, 553–555. [CrossRef] 292. Mubarak, N.M.; Sazila, N.; Nizamuddin, S.; Abdullah, E.C.; Sahu, J.N. Adsorptive removal of phenol from aqueous solution by using carbon nanotubes and magnetic biochar. Nano World J. 2017, 3, 32–37. [CrossRef] 293. Yao, Y.-X.; Li, H.-B.; Liu, J.-Y.; Tan, X.-L.; Yu, J.-G.; Peng, Z.-G. Removal and Adsorption of p-Nitrophenol from Aqueous Solutions Using Carbon Nanotubes and Their Composites. J. Nanomater. 2014, 2014, 1–9. [CrossRef] 294. Diaz-Flores, P.E.; López-Urías, F.; Terrones, M.; Rangel-Mendez, J.R. Simultaneous adsorption of Cd2+ and phenol on modified N-doped carbon nanotubes: Experimental and DFT studies. J. Colloid Interface Sci. 2009, 334, 124–131. [CrossRef] 295. Tóth, A.; Tör˝ocsik,A.; Tombácz, E.; László, K. Competitive adsorption of phenol and 3-chlorophenol on purified MWCNTs. J. Colloid Interface Sci. 2012, 387, 244–249. [CrossRef] 296. Arasteh, R.; Masoumi, M.; Rashidi, A.M.; Moradi, L.; Samimi, V.; Mostafavi, S.T. Adsorption of 2-nitrophenol by multi-wall carbon nanotubes from aqueous solutions. Appl. Surf. Sci. 2010, 256, 4447–4455. [CrossRef] 297. Kassem, A.; Ayoub, G.M.; Malaeb, L. Antibacterial activity of chitosan nano-composites and carbon nanotubes: A review. Sci. Total Environ. 2019, 668, 566–576. [CrossRef] 298. Smith, S.C.; Rodrigues, D.F. Carbon-based nanomaterials for removal of chemical and biological contaminants from water: A review of mechanisms and applications. Carbon N. Y. 2015, 91, 122–143. [CrossRef] 299. Brady-Estévez, A.S.; Kang, S.; Elimelech, M. A Single-Walled-Carbon-Nanotube Filter for Removal of Viral and Bacterial Pathogens. Small 2008, 4, 481–484. [CrossRef] 300. Lu, C.; Su, F. Adsorption of natural organic matter by carbon nanotubes. Sep. Purif. Technol. 2007, 58, 113–121. [CrossRef] 301. Kang, S.; Pinault, M.; Pfefferle, L.D.; Elimelech, M. Single-Walled Carbon Nanotubes Exhibit Strong Antimicrobial Activity. Langmuir 2007, 23, 8670–8673. [CrossRef] 302. Ihsanullah; Asmaly, H.A.; Saleh, T.A.; Laoui, T.; Gupta, V.K.; Atieh, M.A. Enhanced adsorption of phenols from liquids by aluminum oxide/carbon nanotubes: Comprehensive study from synthesis to surface properties. J. Mol. Liq. 2015, 206, 176–182. [CrossRef] 303. Bohonak, D.; Zydney, A. Compaction and permeability effects with virus filtration membranes. J. Memb. Sci. 2005, 254, 71–79. [CrossRef] 304. Mostafavi, S.T.; Mehrnia, M.R.; Rashidi, A.M. Preparation of nanofilter from carbon nanotubes for application in virus removal from water. Desalination 2009, 238, 271–280. [CrossRef] 305. Savage, N.; Diallo, M.S. Nanomaterials and Water Purification: Opportunities and Challenges. J. Nanopart. Res. 2005, 7, 331–342. [CrossRef] 306. Li, Q.; Mahendra, S.; Lyon, D.Y.; Brunet, L.; Liga, M.V.; Li, D.; Alvarez, P.J.J. Antimicrobial nanomaterials for water disinfection and microbial control: Potential applications and implications. Water Res. 2008, 42, 4591–4602. [CrossRef] 307. Nepal, D.; Balasubramanian, S.; Simonian, A.L.; Davis, V.A. Strong Antimicrobial Coatings: Single-Walled Carbon Nanotubes Armored with Biopolymers. Nano Lett. 2008, 8, 1896–1901. [CrossRef] Sustainability 2021, 13, 5717 54 of 54

308. Cortes, P.; Deng, S.; Smith, G.B. The Adsorption Properties of Bacillus atrophaeus Spores on Single-Wall Carbon Nanotubes. J. Sens. 2009, 2009, 1–6. [CrossRef] 309. Ong, Y.T.; Ahmad, A.L.; Zein, S.H.S.; Tan, S.H. A review on carbon nanotubes in an environmental protection and green engineering perspective. Braz. J. Chem. Eng. 2010, 27, 227–242. [CrossRef] 310. Yuan, W.; Jiang, G.; Che, J.; Qi, X.; Xu, R.; Chang, M.W.; Chen, Y.; Lim, S.Y.; Dai, J.; Chan-Park, M.B. Deposition of Silver Nanoparticles on Multiwalled Carbon Nanotubes Grafted with Hyperbranched Poly(amidoamine) and Their Antimicrobial Effects. J. Phys. Chem. C 2008, 112, 18754–18759. [CrossRef] 311. Morones, J.R.; Elechiguerra, J.L.; Camacho, A.; Holt, K.; Kouri, J.B.; Ramírez, J.T.; Yacaman, M.J. The bactericidal effect of silver nanoparticles. Nanotechnology 2005, 16, 2346–2353. [CrossRef] 312. Ihsanullah; Laoui, T.; Al-Amer, A.M.; Khalil, A.B.; Abbas, A.; Khraisheh, M.; Atieh, M.A. Novel anti-microbial membrane for desalination pretreatment: A silver nanoparticle-doped carbon nanotube membrane. Desalination 2015, 376, 82–93. [CrossRef] 313. Nie, C.; Yang, Y.; Cheng, C.; Ma, L.; Deng, J.; Wang, L.; Zhao, C. Bioinspired and biocompatible carbon nanotube-Ag nanohybrid coatings for robust antibacterial applications. Acta Biomater. 2017, 51, 479–494. [CrossRef] 314. Morsi, R.E.; Alsabagh, A.M.; Nasr, S.A.; Zaki, M.M. Multifunctional nanocomposites of chitosan, silver nanoparticles, copper nanoparticles and carbon nanotubes for water treatment: Antimicrobial characteristics. Int. J. Biol. Macromol. 2017, 97, 264–269. [CrossRef] 315. Chi, M.-F.; Wu, W.-L.; Du, Y.; Chin, C.-J.M.; Lin, C.-C. Inactivation of Escherichia coli planktonic cells by multi-walled carbon nanotubes in suspensions: Effect of surface functionalization coupled with medium nutrition level. J. Hazard. Mater. 2016, 318, 507–514. [CrossRef] 316. Al-Hakami, S.M.; Khalil, A.B.; Laoui, T.; Atieh, M.A. Fast Disinfection of Escherichia coli Bacteria Using Carbon Nanotubes Interaction with Microwave Radiation. Bioinorg. Chem. Appl. 2013, 2013, 1–9. [CrossRef]